Why Does the Side of My Abdomen Hurt? Common Causes

Pain along one side of your abdomen can stem from dozens of different organs, muscles, and nerves, all packed into a surprisingly small space. The side that hurts, the exact location (upper versus lower), how the pain started, and what makes it better or worse all narrow the list of possibilities considerably. Some causes are harmless and self-limiting, like a side stitch during a run or trapped gas. Others demand same-day medical attention. Understanding the common causes by region and pattern can help you figure out when to wait things out and when to get to a doctor.

Right Lower Quadrant Pain

When pain parks itself in the lower right part of your abdomen, appendicitis is the diagnosis most people think of first, and for good reason. The appendix sits in that corner, and when it becomes inflamed, the classic pattern is vague pain near the navel that migrates over several hours to the lower right side. Not everyone follows the textbook script, though. The pain sometimes starts right where it ends up, or it may radiate into the back or down the leg. Fever, nausea, and a noticeable worsening when you press and then release the area are common companions. A low blood pressure reading and a high shock index are strong indicators that appendicitis may be present and potentially complicated.¹

Kidney stones can also announce themselves in the right lower quadrant. A stone that forms in the kidney and works its way down the ureter often produces sharp, wave-like pain that starts in the flank and radiates forward and downward. A case report of a 52-year-old man describes exactly this pattern: acute right flank and lower quadrant pain caused by a ureteric stone with associated swelling of the kidney above the blockage.² Unlike appendicitis, kidney-stone pain tends to make you restless. You cannot find a comfortable position, and you may feel the urge to urinate frequently even though little comes out.

Right Upper Quadrant Pain

The right upper corner of your abdomen is gallbladder territory. Gallstones are extremely common, and most of the time they sit quietly and never cause trouble. When a stone blocks the duct leading out of the gallbladder, though, you get biliary colic: a squeezing pain in the right upper quadrant that often radiates around to the back and can last up to twelve hours.³ The pain frequently shows up after a fatty meal, peaks within an hour, and then gradually eases. If the pain persists beyond twelve hours or comes with a high fever and yellowing of the skin, the situation may have escalated from simple colic to cholecystitis or a bile duct obstruction, both of which typically require hospital care.

Left Lower and Left-Side Pain

On the left side, the sigmoid colon is the usual suspect. Diverticulitis, where small pouches in the colon wall become inflamed or infected, is sometimes described as “left-sided appendicitis” because the location and severity can feel similar. Compared to a less well-known condition called primary epiploic appendagitis (a benign inflammation of small fat-filled pouches on the colon surface), diverticulitis tends to produce more widespread tenderness across the left side, is more often accompanied by fever, and is more likely to show elevated white blood cell counts. In one comparative study, fever was present in 40% of diverticulitis cases versus only about 7% of epiploic appendagitis cases, and rebound tenderness was also far more common with diverticulitis.⁴ Epiploic appendagitis, by contrast, tends to cause a more pinpoint tenderness in the left lower quadrant and resolves on its own with anti-inflammatory medication. The distinction matters because diverticulitis sometimes requires antibiotics or even surgery, while epiploic appendagitis almost never does.

Left Upper Quadrant Pain and the Spleen

Left upper quadrant pain is less common but has its own set of causes. The spleen lives just under your left rib cage, and enlargement or injury to it can produce a dull ache or sudden sharp pain in that region. In people with infectious mononucleosis (caused by Epstein-Barr virus), the spleen can swell significantly, and in rare cases it can tear even without trauma. A hallmark of splenic injury is referred pain to the left shoulder, known clinically as Kehr’s sign, caused by irritation of the nerves under the diaphragm.⁵ If you have mono and develop left-side abdominal or shoulder pain, that combination warrants an immediate trip to the emergency department.

Another underappreciated source of left upper quadrant discomfort is splenic flexure syndrome, where excessive gas or stool accumulates at the sharp bend in the colon near the spleen. This produces bloating, distension, and cramping pain that can mimic more serious conditions but is essentially a motility problem.⁶ The clue is that the pain tends to improve with passing gas or having a bowel movement and worsens with large meals.

Kidney Stones and Flank Pain on Either Side

While kidney stones were mentioned earlier in the context of right-sided pain, they are just as common on the left. The pain pattern is essentially the same regardless of the side: intense, colicky discomfort starting in the flank and sweeping forward toward the groin. Bedside ultrasound is now a first-line tool in emergency departments for evaluating this type of pain. It detects hydronephrosis (the backup of urine above a blockage) with reasonable accuracy and helps doctors rule out other emergencies like abdominal aortic aneurysm or internal bleeding at the same time.⁷ If you have ever had a kidney stone, you already know the sensation. If you have not, the intensity of the pain can be alarming enough to make people suspect something far worse.

Gynecological Causes

In women, the ovaries and fallopian tubes sit low in the pelvis, and problems with either can produce pain that feels like it is coming from the side of the abdomen. Ovarian cyst accidents, including torsion (twisting), internal bleeding, and rupture, are well-known causes of acute one-sided pelvic pain.⁸ The pain can come on suddenly and be severe enough to mimic appendicitis or kidney stones, especially when the cyst is on the right side. Ultrasound is often enough to pin down the diagnosis and guide whether the situation needs surgery or can be managed conservatively.⁹

Ectopic pregnancy, where a fertilized egg implants outside the uterus (usually in a fallopian tube), is a more dangerous possibility. It typically produces one-sided lower abdominal pain with irregular vaginal bleeding. Ruptured ectopic pregnancy can cause life-threatening internal hemorrhage, so it is always on the list of diagnoses emergency physicians consider in women of reproductive age who present with sudden abdominal pain.¹⁰ Cervical motion tenderness on examination is one of the strongest clinical indicators that ectopic pregnancy may be the cause.¹

Cyclical causes are worth noting too. Endometriosis can produce recurring pain that lines up with the menstrual cycle and tends to favor one side depending on where the tissue implants have settled. Mittelschmerz, or mid-cycle ovulation pain, is a milder and very common example of one-sided lower abdominal discomfort that lasts a few hours to a day and alternates sides from month to month.⁸

Musculoskeletal and Nerve Pain

Not all abdominal pain comes from inside the abdomen. The abdominal wall itself has muscles, nerves, and ribs that can be the source of the problem. Slipping rib syndrome is an underdiagnosed condition where the cartilage connecting one of the lower ribs loosens, allowing the rib to shift and irritate the nerve running beneath it. The pain typically comes on suddenly with a jerking or twisting motion and can be localized to the side, radiate around the chest wall, or even produce a diffuse deep aching that mimics organ pain.¹¹ The giveaway is that the pain worsens with certain movements, deep breaths, or direct pressure on the rib, and it does not come with any of the systemic signs like fever, changes in bowel habits, or blood in the stool that would point toward an internal cause.

Shingles is another cause of side-of-the-abdomen pain that can fool both patients and doctors. The varicella-zoster virus reactivates along a nerve root and typically produces burning pain in a band-like pattern along one side. The tricky part is that the pain can precede the characteristic rash by several days, during which time the symptoms may look exactly like an acute abdominal emergency.¹² In people with weakened immune systems, the abdominal pain from a disseminated varicella-zoster infection can be especially severe and may appear before any skin lesions at all.¹³ If you develop a burning, one-sided abdominal pain that does not fit a clear pattern and then notice small blisters appearing on the skin in that area a few days later, shingles is likely the answer. Even in pregnancy, shingles has been documented as a cause of puzzling left upper quadrant pain before the rash appeared.¹⁴

The Exercise Side Stitch

If the pain in your side only shows up during physical activity, you are probably dealing with exercise-related transient abdominal pain, commonly called a side stitch. This is one of the most frequent complaints among runners and swimmers and tends to hit the right side more often than the left. Research examining side stitches triggered by running after drinking fluids found that the pain could not be explained by changes in blood flow to the digestive organs. Instead, the evidence pointed toward the fluid-filled gut tugging on the ligaments that suspend the abdominal organs, particularly when the body is bouncing up and down during running.¹⁵ Slowing your pace, exhaling forcefully, and pressing gently on the painful spot usually resolves the stitch within minutes. Avoiding large volumes of fluid or food within an hour or so of exercise can help prevent it.

When the Pain Starts Somewhere Else

One of the more counterintuitive aspects of abdominal pain is that the source may not be in the abdomen at all. Lower lobe pneumonia, for example, can irritate the underside of the diaphragm and produce pain that the brain interprets as coming from the upper abdomen. This has led to cases where patients with pneumonia ended up in the operating room for suspected surgical emergencies before the real cause was identified.¹⁶ A cough, shortness of breath, or recent respiratory illness preceding the abdominal pain can be a useful clue.

In young men, testicular torsion is another condition that occasionally presents with abdominal pain rather than the expected groin pain. In a study of 76 cases of testicular torsion, about 12% initially presented with acute abdominal pain rather than scrotal symptoms. In all those cases, the abdominal pain was on the same side as the twisted testicle and often started abruptly during sleep.¹⁷ This is a time-sensitive emergency because the testicle loses blood supply, so any young male with sudden lower abdominal pain should be examined for testicular torsion even if the pain seems to be entirely in the belly.

Irritable Bowel Syndrome and Visceral Hypersensitivity

When no structural cause is found, irritable bowel syndrome (IBS) is the most common diagnosis. IBS is characterized by recurrent abdominal pain linked to changes in bowel habits, whether that means diarrhea, constipation, or both. The pain can land anywhere in the abdomen but frequently favors the lower quadrants, especially the left. A key feature is that the pain often improves with a bowel movement and worsens with stress or certain foods.

A central concept in understanding why IBS hurts is visceral hypersensitivity: the gut’s nerve endings are essentially turned up too high, so normal stretching and movement of the intestines registers as pain. This process can involve both the nerves in the gut wall and the way the brain processes signals coming from the digestive tract.¹⁸ The practical implication is that normal diagnostic tests (blood work, imaging, endoscopy) often come back completely clean. That does not mean the pain is imaginary. The signaling system itself is the problem, and treatments increasingly aim at calming those overactive pathways rather than looking for a structural abnormality that is not there.

Vascular Causes

In older adults or people with cardiovascular risk factors, ischemic colitis is a cause of side abdominal pain that deserves mention. It happens when blood flow to part of the colon drops temporarily, usually due to low blood pressure, a blood clot, or narrowing of the mesenteric arteries. The most common presentation is abdominal discomfort accompanied by bloody diarrhea.¹⁹ The pain tends to affect the left side because the splenic flexure and descending colon are the segments most vulnerable to blood flow disruptions. Any episode of abdominal pain combined with blood in the stool in an older adult warrants prompt medical evaluation.

Red Flags That Mean You Should Not Wait

Most episodes of side abdominal pain resolve on their own or turn out to be something manageable. But certain features suggest a situation that could deteriorate quickly. You should seek immediate medical attention if the pain is accompanied by any of the following:

• Fever above 101°F (38.3°C): This points toward an infection or inflammatory process that may need antibiotics or surgical intervention.
• Rigid abdomen: An abdomen that feels hard and is extremely painful to touch could indicate peritonitis, meaning something has perforated or ruptured inside.
• Bloody stools or vomit: Blood loss from the gastrointestinal tract can indicate ischemia, severe ulceration, or a structural emergency.
• Lightheadedness or fainting: These suggest significant blood loss or a drop in blood pressure, both of which may be signs of internal bleeding.
• Pain that wakes you from sleep: Pain intense enough to disrupt sleep is less likely to be functional and more likely to reflect an organic cause.
• Positive pregnancy test with one-sided pain: Until an ectopic pregnancy has been ruled out by ultrasound, this combination is treated as a potential emergency.

Systematic reviews of clinical red flags for abdominal pain confirm that signs of hemodynamic compromise, like low blood pressure and a high ratio of heart rate to blood pressure, are among the strongest predictors of serious underlying disease in people presenting with acute abdominal pain.¹ When in doubt, erring on the side of getting checked is always the safer call. An unnecessary emergency room visit is a minor inconvenience compared to a missed surgical emergency.