I'm having a really bad stomach ache. What do you think could be the cause?

Well, for the past two nights (nights only, for some reason) I’ve been having a pretty bad stomach ache. I thought last night was a one time thing (stomach pains so bad I was crying, couldn’t move very well) but now it’s back tonight. Any ideas as to what it could be? I find it very curious that it only happens at night though.