cheated on me :(

I'm sorry you have to go through this ordeal, there is no excuse for adultery and this is completely unfair to you. I suggest you should wait a week or two to see if he's going to come clean about it. If he does, then at least you know that he is finally being honest about his mistake and is actually trying to salvage your marriage. But if he doesn't, then ask him directly. Tell him exactly what you saw and demand an explanation. Make sure you're emotionally prepared for whatever answer he gives you. Good luck.