Possible causes can be:
- Muscle problems: There is a possibility that your muscles are still repairing themselves from the previous training session/exercise. If you attempt to work your abdominal muscles too soon after a recent routine without giving the muscles a chance to repair themselves, those muscle fibers will continue to tear, causing discomfort.
- Reduced air supply: Due to lack of oxygen supply to the muscles, it is possible that you may get abdominal cramps or pain especially when you are doing cardiovascular activity or weight lifting. Simply relax and slow down.
- Gastrointestinal problems: Sometimes during periods, blood is routed away from digestive system and food in stomach does not get digested properly hence causing pain. Try not to eat anything heavy 2 hours before your workout program and make sure you hydrate yourself enough before exercising.