I was in an auditorium with my girl yesterday. It started off as a good time; we were laughing, jesting, messing with each other. Randomly she starts getting distant. The group we were with won an award and we stood up to clap and yell, and I went to high five her. Planning to dodge her high five and pick her up, I dropped my hand to feign the high 5. Immediately, with her other hand she slaps me in the face. Still in shell shock, I looked at her incredulously and said "Don't ever slap me again". She immediately gets defensive and says that I slapped her before and that its okay, but I have never slapped her in my life. I playfully karate chop her throat when we're playing around, and even that is softly, but I have never laid a hand on a girl out of anger. I remain adamant and say that she is lying because I never slapped her before, but she sticks with her reasoning. I refuse to start bickering at this event and instead talk to my friend and have fun. What should I do about this to resolve it in the best way?

Edit: I called her that night for an explanation and she apologized and said it was uncalled for and will never happen again. AWALT. She said she had other things on her mind and took it out on me, and she will proactively try not to repeat it. I still feel like my gut is telling me that I should do something more about getting slapped but I'm not sure if I'm overreacting or not, and if it would be overboard to keep pressing.