Factors that may cause IBS include severe infection, inflammation, constipation (hard stools), and abnormalities in the nerves in your digestive system. The walls of the intestines are lined with layers of muscle that contract as they move food through the intestines. Stronger contractions (spasms) that last longer than normal can cause gas, bloating and diarrhea. Weak intestinal contractions can slow food passage and lead to hard stools (constipation), which can also lead to IBS.
Irritable bowel syndrome (IBS) is a common disorder that affects the large intestine. The exact cause of IBS isn't known. It is believed weak intestinal contractions can slow food passage and lead to hard, dry stools causing IBS. Inflammation in the intestines can also lead to IBS.