Is my bf cheating?

Usually I trust my man but things have been weird lately and idk what to think. Today i found a nail in his car, like an acrylic nail that had been fully taken off, in the front seat (not mine, i never wear acrylics i hate them). I asked him about it and he was like wtf is that, that's disgusting, you know i'd never cheat, then blamed it on his mate who apparently drove his car with a girl in it. Normally i'd believe him but the past few weeks he's been going out on weekends and not coming home until 5 or 6am. What do you ladies (and men if there are any) think? Am i being stupid for wanting to believe him or am i an idiot for believing him all these times something suss has happened?