If you haven't ridden in a few months this might not be relevant, but there were a couple of times that I had really bad stomach cramps because I'd eaten something heavier than usual for a bike ride and the weather was cool and I didn't have enough layers on. Basically I just didn't have enough blood to keep warm and digest at the same time, so the digestion suffered. Trying to do a third thing (pedal a bike) made it that much worse.