Irritable bowel syndrome (IBS) - the walls of the intestines are lined with layers of muscle that contract and relax as they move food from the stomach through the intestinal tract to the rectum. Normally, these muscles contract and relax in a coordinated rhythm. If IBS is present, the contractions may be stronger and last longer than normal. Food is forced through the intestines more quickly, causing gas, bloating and diarrhea. In some cases, however, the opposite occurs. Food passage slows, and stools become hard and dry.  People with IBS may have a heightened sensitivity to stretching of the bowel with gas leading to pain or bloating. Others believe the central nervous system may affect the colon. And because women are twice as likely to have IBS, researchers believe that hormonal changes also play a role.  Many women find that signs and symptoms are worse during or around their menstrual periods.