Did he cheat on me or is it just me?

You should give him another chance. Yes, he kind of cheated on you. He cheated mentally, not physically. You two really seem to care about eachother and he's a guy, sometimes they just do things we females don't like and don't understand. Im not giving him an excuse, because there is no real excuse for that, but if he apologizes and stuff, as long as he doesn't do it again, you two can probably forget about it and move on. But, if I were you, i'd keep him far, far away from that girl.