<human>: Last night my girlfriend said she needed to tell me something so we went and found someplace private to sit. 
I could tell something was wrong. She told me that over our last school break (boarding school), she had been hanging out with another guy [15M] and then he had pushed her down (sounded like it was in a playful way) and kissed her. 

She claimed to not like him and said that he didn't know she was dating me (and still doesn't). She said she felt really guilty because she didn't do anything to stop him and also that she didn't have the courage to tell me before. She said she hadn't told anyone, even her best friends which she shares everything with. She also said she could understand if I never wanted to see her again. She said the kiss was about 1 month ago, and she and I had only kissed for the first time about a week earlier, but we've been in a relationship for 9-ish months. 

I didn't really know what to say at all. We hung out together the rest of the night and had fun, but there was definitely something awkward. I feel like I need to say something more but I don't know what. I'm also just confused because our relationship had been amazing up until now.

Could you distill the essence of the text into 1-3 sentences?
<bot>: My girlfriend was kissed by another guy a month ago and didn't tell me until last night. I don't know what to do now.