Irritable bowel syndrome (IBS) is a common disorder that affects the large intestine. It can cause abdominal pain, bloating, gas, and changes in bowel habits. The exact cause of IBS is unknown but it may be related to overactivity of the nerves controlling the muscles in the gastrointestinal tract. This overactivity can lead to cramping or spasms in the intestines which causes discomfort and irregular bowel movements.