This is an update to this post: http://www.reddit.com/r/AskReddit/comments/kkzfo/a_random_girl_has_contacted_me_saying_that_my/ First of all, I want to thank everyone for their responses. I read every response and message, and took all of it to heart. Here is what happened next:

The morning after posting to Reddit, I showed my boyfriend the message. He denied everything, but I could tell right away from his body language that something had happened between him and the girl. I asked him if he was okay with me talking with the girl in person or contacting the host of the party to ask about the girl's story. At first, he was really reluctant to let me investigate further, but after awhile, he seemed resigned to the fact that I was going to get to the bottom of this one way or another. He told me that he understood why I was worried and that he was okay with me doing anything I could to set my mind at ease. I took that as good sign and called the host of the party (who really didn't tell me much, just that my boyfriend and the girl were both there). I then arranged an in-person meeting with the girl.

I met the girl for coffee. Once she was in front of me and I asked her face-to-face about whether she got a rape kit done, she changed her story a lot. She told me she had met my boyfriend at the party last Friday. She said that they spent the whole night drinking and talking. She said that after the party was over, she and my boyfriend went back to her apartment and "ended up accidentally having sex" (whatever the fuck that means). She said she sent the message to me the next morning because she was upset when she looked him up on Facebook to friend him and saw that he had a long-term girlfriend. He apparently did not mention me once the entire night and she was enraged that he "used her for sex" like that. I told her that I thought that was more than just a little crazy and she agreed, claiming that "her crazy side just came out" and she couldn't help but send me the message. I told her that it was very, very wrong to make false rape accusations and that she should be completely ashamed of herself. She also said that she was really sorry for everything, and that she would not contact me or my boyfriend again.

Needless to say, I went home and confronted my boyfriend. He admitted to everything and told me the exact same story as the girl. We broke up, and he is in the process of moving out of our apartment. I am devastated because I really loved and trusted this man, but I have too much self respect to continue to deal with a cheater. I do not believe anyone has sex by accident, and being really, really drunk just isn't a good enough excuse for me. Anyway, thanks for everything, Reddit, and don't worry, I am getting tested for STDs tomorrow.

tl;dr: My boyfriend and the girl in question both admitted that they had consensual, drunk sex. The girl sent the message because she was angry my boyfriend didn't tell her he had a girlfriend. I broke up with my boyfriend.