IBS is a functional disorder. Even though the digestive tract looks normal, it doesn't function as it should. Muscles in the intestines move food from the stomach to the rectum. Normally, they contract and relax in a gentle rhythm that moves the food along in a fairly predictable schedule. But with some people, the muscles in the intestines spasm. That means the contractions are longer and stronger than normal. Those spasms are painful. They also disrupt the movement of food through the intestines. If they slow it down, you become constipated. If they cause it to move through too quickly, you get diarrhea. It's not unusual for people to alternate between the two. Another cause of discomfort for people with IBS results from oversensitive nerve endings in the digestive tract. Small bubbles of gas that wouldn't bother most people might be quite painful for you. Your heightened sensitivity can also lead to swelling and bloating.