Most Helpful Guy
Maybe he bought the bartender a shot to be friendly, but told her to not tell you because he didn't want her to say, "Hey, your boyfriend bought me a shot," because that would have been soooooo much worse coming from her. Since he told you, it obviously didn't mean anything to him so far as relationships go, so I'd just let it go this time. He wasn't trying to cheat, he was just being nice to another girl.
Most Helpful Girl
I would be upset. It's the fact that he told her to keep it from you that's the worst... I think that's disrespectful. I'm sorry this happened to you. :( have you talked to him about it yet? I would ask him to explain himself. Both why he did it and why he tried to keep it a secret from you.