A few times, each day, strong muscle contractions move down the bowel pushing fecal material ahead. Some of these strong contractions result in a bowel movement. In a normal person, these contractions are smooth and rhythmic.

When IBS occurs, the colon seems to contract in a disorganized, at times violent, manner. These abnormal contractions result in changing the bowel patterns.
The localized areas of the colon may remain contracted for a prolonged time or intestines may move very slowly. When this occurs, stools are retained for prolonged period in the intestines and become dry. The cumulative effect is that the person suffers from constipation.

Also, air may accumulate behind these localized contractions, causing the bowel to swell. So bloating and abdominal distress may occur.

In others, these contractions take place in so rapid succession and move the contents of intestine so fast, that the bowel does not get enough time to absorb the water from the digested matter, resulting in watery diarrhea.

A second major feature of IBS is the abdominal discomfort or pain. These strong, sharp contractions cause excruciating pain. Moreover, the intestines in people with IBS often send heightened pain signals to the brain, so people with IBS can feel extreme pain after a normal meal, during a normal bowel movement, or even with a little bit of gas.

Mucous is a normal secretion of the bowel, although most of the time it cannot be seen. IBS patients sometimes produce large amounts of mucous, but this is not a serious problem.