1. although these exercises are good for strengthening and toning of abdominal muscles,but the stress (pressure) they put on the muscles and digestive tract causes pain. 2. probably you might have over exert yourself which could have caused a slight tear (abdominal strain)? 3. other possible factors could be your under hydration,electrolyte imbalance,muscle fatigue, and not warming up and stretching before performing sit ups. 4.in the mean time some preventive measures can be taken like; eating meals two hours prior to do sit ups, avoid caffeine, Vitamin C, alcohol,carbonated beverages, sugar and fat, keep yourself well hydrated by consuming fresh fruits and vegetables. 5. it is better to get yourself checked by your health care provider or certified trainer for accurate diagnosis and evaluation