A pain in your stomach is a bellyache. Don't eat too much cotton candy at the fair, or you'll definitely get a bellyache!
You might also refer to a bellyache as a tummy ache or an upset stomach. Whatever you call it, it's no fun, and it might even cause you to lie in bed and moan a little. As a verb, bellyache means "complain" or "whine." If your brother insists on taking part in a hotdog eating contest, you might be stuck listening to him bellyache about his bellyache later.