Many possibilities. Abdominal cramping during abdominal exercises occurs with fatigue or overtaxing of the associated muscles. They may be deconditoned, and the cramping will eventually subside as you get in better shape. Cramping during heavy exertion, especially during hot/humid environments, can be a sign of dehydration and/or sodium or potassium depletion. Cramping can also occur due to nonorthopedic condtions.