Left-sided pain during a bowel movement usually comes from the sigmoid colon, the S-shaped segment of your large intestine that sits in your lower left abdomen. This is the last stretch stool travels through before reaching your rectum, and it’s where pressure builds highest during a bowel movement. Several conditions can make that normal pressure painful, ranging from simple constipation to inflammation that needs treatment.
Your Sigmoid Colon Takes the Most Pressure
Understanding the basic plumbing helps explain why pain shows up on the left side specifically. Your large intestine frames your abdomen in a rough rectangle, and its final segment, the sigmoid colon, curves down into your pelvis on the left. Because the sigmoid has the smallest diameter of any section of the colon, it generates the highest internal pressures when it contracts to move stool along. During a bowel movement, those contractions intensify. If anything in that area is inflamed, stretched, or blocked, you feel it.
Constipation and Stool Backup
The most common and least worrisome cause is simply having hard or backed-up stool sitting in the sigmoid colon. When stool dries out and compacts, it presses against the intestinal wall and creates a sensation of bloating, abdominal pain, and lower back discomfort. Trying to push that stool out increases the pressure even further, which is when the pain spikes. In severe cases, this becomes fecal impaction, where the mass is too large or firm to pass on its own. Increasing fiber, fluids, and physical activity usually resolves mild constipation before it reaches that point.
Diverticulitis
Diverticula are small pouches that form where the colon wall is naturally weakest, specifically at the points where blood vessels pass through the muscular layer. As you age, the collagen in your colon wall develops more cross-links, making it stiffer and less elastic. That stiffness makes the wall more prone to tiny tears and herniations, and the sigmoid colon is the most common site because of those higher internal pressures.
Most people with diverticula never have symptoms. But when one of those pouches becomes inflamed or infected (diverticulitis), it causes a steady, often severe pain in the lower left abdomen that gets worse with movement and straining. A bowel movement forces the sigmoid to contract around the inflamed pouch, intensifying the pain. You may also notice fever, nausea, or a change in bowel habits. Mild diverticulitis is often treated at home with a temporary change in diet and antibiotics, while complicated cases may need hospital care.
Irritable Bowel Syndrome and Visceral Hypersensitivity
IBS can make normal intestinal activity genuinely painful. The key mechanism is something called visceral hypersensitivity: your threshold for sensing pain in internal organs is lower than usual. Normal amounts of gas, fluid, or stool moving through the colon create discomfort that most people wouldn’t notice. Researchers can measure this by applying small amounts of pressure inside the colon during testing. Most people feel nothing, but those with visceral hypersensitivity report real pain from the same stimulus.
Because the sigmoid colon is both the narrowest segment and the one doing the most work during a bowel movement, it’s often where IBS pain concentrates. You might notice the pain is worse during periods of stress, after certain foods, or when your bowel habits shift between diarrhea and constipation. IBS doesn’t damage the colon, but the pain is not imaginary. It reflects a real difference in how your nervous system processes signals from your gut.
Proctitis and Inflammatory Bowel Disease
Inflammation of the rectum (proctitis) or the lower colon can cause left-sided pain that flares during bowel movements. Ulcerative colitis, for example, often starts in the rectum and extends upward into the sigmoid colon. Symptoms include rectal bleeding, mucus in the stool, a constant urge to have a bowel movement even when nothing comes out, and pain on the left side of the abdomen. The act of passing stool through inflamed tissue makes the pain worse.
Proctitis can also result from infections, radiation therapy, or certain medications. If you’re seeing blood or mucus alongside your pain, that points more toward an inflammatory cause than a functional one like IBS.
Pelvic Floor Dysfunction
Sometimes the problem isn’t the colon itself but the muscles around it. Dyssynergic defecation is a condition where the muscles and nerves in your pelvic floor don’t coordinate properly during a bowel movement. Instead of relaxing to let stool pass, the muscles tighten or fail to open at the right time. This creates excessive straining, incomplete evacuation, and pain that can radiate through the lower abdomen. Because the sigmoid colon sits deep in the pelvis, pelvic floor dysfunction can produce pain that feels like it’s coming from the left side. Specialized physical therapy focused on the pelvic floor is the primary treatment and has good success rates.
How Doctors Figure Out the Cause
If your pain is occasional and clearly linked to constipation, dietary changes are a reasonable first step. But persistent or worsening left-sided pain during bowel movements usually calls for some form of evaluation. Ultrasound is increasingly used as a first-line imaging tool for suspected diverticulitis. European expert guidelines recommend it as a routine procedure for anyone with suspected diverticulitis, noting that it can confirm the diagnosis and help assess severity early. A CT scan may follow if the ultrasound is inconclusive or if complications are suspected. For symptoms suggesting IBS or inflammatory bowel disease, a colonoscopy gives a direct view of the colon lining.
Signs That Need Prompt Attention
Some patterns suggest something more than routine digestive discomfort. Blood in your stool, fever or chills, pain that has gradually worsened over weeks, vomiting blood, an inability to pass gas or stool at all, or yellowing of the skin or eyes all warrant a call to your doctor. Severe pain that doesn’t let up, even when you’re not having a bowel movement, is another signal not to wait.