Girlfriend told me a guy kissed her at the bar... Is she leaving out details?

So, a few months ago, my girlfriend went out with her slutty single friend, not being judgmental, she just really is that way. Anyway, she went to this big fratty bar with her, got all dressed up, and I'm assuming danced with some dudes though she said she only danced with her friend. Whatever I'm pretty trusting.

When she first told me about the night she mentioned some guy tried to kiss her but she stopped him. But parts of the story didn't add up, but she kept saying that's all that happened. I dropped it.

Now months later, after making up after an argument. I brought it up again, and said I still didn't believe her. That's when she admitted he did kiss her. But said she wasn't flirting, just being nice, and she stopped him right away... Needless to say, since she lied about it, it makes me think that more actually happened. She apologized, and to her credit she hasn't been hanging with that girl like that anymore, and has stopped drinking for the most part.

What do I do? She said I had a "freebie" to kiss another girl, but she had to be there, she didn't seem serious though. Like it was a fake offer to make me feel bad or something. But also since she offered that up, it makes me think she is still hiding something she did that could be more serious. She did stay at that friends house that night. That being said I trust her for the most part, and I don't think she slept with anyone. What do I do?

I should mention, I have never cheated on her, and never accuse of stuff like that. On the other hand, if I am even a few minutes late from work she accuses me of cheating. Which makes me think she is actually the one doing stuff, or just really insecure.
Girlfriend told me a guy kissed her at the bar... Is she leaving out details?
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