HELP!! Just wanna get it off my chest

So I kind of ‘hooked up’ with a guy and and after we talked a little about it, he said it was cool and it was ok to do it again- which was fine with me. I heard that he had something with someone else, which was fine I just didn’t want to be messing around with him if he did, so I asked and he FLAT OUT DENIED IT! But then I see them together a week later and idk what to do?! We were friends before but now I just feel weird about the whole thing :(