Help with my boyfriend?

I don't think you're over thinking it.. but i do think that you shouldn't jump to that conclusion. You should ask him what he does in those times.. and then if you feel particularly brave ask him to repeat that backwards. It's hard to remember your lie backwards (if he's lying). But I'd say trust him... if he's never cheated on you before, or never cheated on any other girlfriend he's had then i think you should give him the benefit of the doubt.