Hey. basically everyone had gone out in my flat at uni 3 nights ago. Me and my girlfriend had decided to stay in that night after a long day. The night was ok, we didn't argue but we were kind of quiet/ off with each other for a while - but nothing major.
At about 2am When all my friends got back at the halls where i live they knocked at my door and asked to join them to just chill out and stuff so i went with them whilst my girlfriend went back to her room.
After a few hours of chatting with friends and chilling out at about 4am, i popped back to my girlfriends room (about 10 seconds away) just to see how she was. She said that her friend was coming around (ill call him John) because she was bored and he was bringing a mate (i'll call him Jack). I knew she had already got with this guy before we were together. I thought nothing of it as she says they're just friends now and i trusted her.
At about 6am, Jack came down to the living room where me and my friends were chilling out saying "Oh God, I don't know why John dragged me here, I left the room before I could find out what they were doing." I obviously was starting to get really furious at this point so i went upstairs to my girlfriends room to find the door was locked (she never usually locks her door.) I pressed my ear to the door to hear giggling and whispering and then the sound of kissing. My heart sank. I didn't know what to do. I decided to wait a while in order to make sure that what i heard was not just a fluke so i went away to calm down for about 15 minutes before i confronted them.
I returned to hear the exact same sounds so in a fit of anger i banged on the door furiously. My girfriend opened the door and i asked them both what was going on (i was quite verbally aggressive at this point and they both looked quite scared - i'm about twice the size of both of them put together). At first they both said nothing happened and she told me i was being stupid and told me to leave and locked the door.
I knocked back on the door straight away and asked what had happened saying i heard kissing sounds. The guy didn't know that she had a boyfriend and asked me "are you seeing each other" to which my girlfriend replied "no." Then they both came clean saying that he was trying to get with her and that he was kissing her arm and tickling her whilst lying in bed together. Then he left and my girlfriend apologised and said she had made a mistake and that she loves me. She maintains that he was trying with her and that she just wanted to go to sleep and that the giggling was nervous giggling. I have since talked to her best friend who maintains that as far as she knows they did not kiss or doing anything futher.
I said i wanted to break up with her later that night but have since said i have forgiven her. I'm unsure what to do and who to talk to. Am i right to be this upset or am i overreacting? Should I forgive her ?
Thanks for listening and for your help..