PLEASE HELP- I think he's cheating?

Clearly this is eating at you, so find out the truth directly from him. Don't go snooping around, checking his cell phone or anything, even though all signs point to him possibly cheating on you. Just ask him straight out, that he's been acting weird and it's got you worried otherwise this suspicion is only going to poison your relationship further and that would suck if he isn't even cheating on you.