<human>: This is my first time posting here, or even on this subreddit for that matter, but i was looking thriugh all of the subreddits and saw that this would probably be a good place to ask for advice about this issue.

 I've been in what I've felt has been a very good relationship for the past month or two. It hasn't gotten too serious yet, but we cuddle, go out, and just spend a lot of time together. The girl is someone who I've had strong feelings for for a long time, but it just never seemed to work out due to outside circumstances such as a death in the family and things along those lines. But here we are, both 19 year old college students,, two years after meeting each other and it's finally going somewhere, but tonight something very strange happened that I can not stop thinking about.

The girl who I've been seeing was having a small party at her house, just us and about ten other friends from high school who were in town for Thanksgiving. So we all got to hang out and have a good time and everybody started leaving at about 1:30 in the morning. I was the last one out of the house, but one of the other guys from the party was in his car, waiting in front of the drive way. I thought this was kind of strange but I figured he forgot something and she was gonna bring it out or something. So I went and got in my car, looked through my CDs to find something to listen to and after a couple minutes, I saw my lady friend leave her house and get in this other guys car and they both drove away. 

I've known both of these people for two years at least and the girl definitely does not seem like someone who would lie or cheat or anything, but this is just stressing me out beyond belief. I haven't had anxiety so severe for a long time and I just don't know what to do about it, which is why I'm here now. Any advice that anyone could offer would be massively appreciated. 

Thank you for taking the time to read this and for any advice you may be able to give.

Can you succinctly summarize the text in 1-3 sentences?
<bot>: Something odd is going on between my girlfriend and another friend from highschool. I have no idea what and it's stressing me out.