Strange man ed my girlfriend?

Me and my girlfriend went to an upscale bar the other night and while i was ordering her a white russian and an appletini for myself, some stud walked up and slapped her on the . I was like "Like, Wtf, ok?" And she goes all blushed and starts giggling like a stupid retard. So I got the drinks and sat down and she goes "I'm going to the bathroom" and I go "whatevz" and she took so long I thought maybe she was constipated or something, but after several minutes, I just got up and walked over to there and saw the guy that slapped her kissing her on the mouth. I went to sit back down and when she came back I go "Why were you kissing that guy?" and she goes"Because" and I go "Oh" and she goes "He and I are friends, I know what I'm talking about." I think she's cheating on me.