It was my girlfriend's birthday party with friends. We all had dinner and went out clubbing. For most of the night it was just our circle of friends until a group of guys showed up, friend of a friend thing. Not sure if one of new people knew I was the boyfriend or not but he was flirting with my girlfriend. After a while I noticed them still hanging out, his hands around her wist and his hand in her hair as he was talking to her. At this point I was wanted to leave before I do something I can't take back. I kept looking at them trying to be calm, ironic I know. Then all of a sudden she got mad, he said something to her and it set her off. She came to me and said he was calling her out for not taking a shot. Now I am mad, not just at him but also at her. Within a few seconds his hands were back in her hair as he tried to apologize to her. Of course my girlfriend doesn't remember and her friend that invited the guy said nothing like that happened. She told me I shouldn't be upset because she would never do such a thing. I know you are only seeing it from my viewpoint and not hers, but how should I deal with this?

You left out the vital information about how drunk each of you was.
DROP things like that that happen when you are all drunk.

I was sober. She was drunk.

Alcohol only lowers inhibitions, it doesn't change how someone would act. Blaming alcohol on her actions would be wrong. It just makes it easier for her to do those actions she would otherwise do. Memory loss is associated with too much alcohol as well, was she fall down drunk or just tipsy? You have to be the judge of how drunk she was, and how many mind games you like to play. If she is using memory loss as a defense and you don't think she was that drunk and don't want to play mind games call her out and dump her there are better women out there.

When I was in my twenties I remember dating a woman that wanted to play mind games and I was done with them. The next lady I dated was a little older and I've been with her for 18 years in March.

Good luck dude.