<human>: Can you give me the gist of the text in a nutshell?

Long story short: 

We were at a party, I had to leave early (1 am) because I had a family event early the next day. Some guy initiated a kiss on her and they kissed for a short while - she called me (3 am) and told me about it and I got angry and felt really let down so I told her Id rather talk about it the next day.

She said she would never cheat on me etc etc. But we have only been toghether ~3 months. I care about this girl but its my first serious relationship so should I just let it go now?

How should I proceed?
<bot>: Guy initiated short kiss on my gf and she went along. Later that night she tells me about it and feels bad. What do?