What do you do when you catch your boyfriend cheating on you?

All you need to do is confront him the next day and address the issue. Speak in a calm voice and ask him why he did this. If he says that your not treating him as he should be, and the other girl does. Then you need to respect that. Cheating might be wrong, but if he had a reason other than a stupid one then you're probably the reason he's doing it.