<human>: I, (f/20), have been dating my boyfriend (21) for a year and eight months now.  We normally don't have many problems, but recently his past has been bothering me a bit.  We were talking on Skype recently (he lives about an hour away) and came upon the subject of a particular YouTube video.  He mentioned that he really disliked it, and I asked him why.  He said that he once hooked up ("making out," not sex) with a girl who was obsessive about the video.  He called her a slut because she hooked up with a lot of guys.  I asked him who the girl was and he responded, "It was Rebecca (name changed).  Oh, you met her once, remember?"

Now, a few things came to my mind at the time we were chatting.  First, the way he said it so casually, "Oh, you met her once."  I understand that it's not really my place to get jealous over something that happened a long time ago, but it still made me feel betrayed by the fact that I wasn't given full disclosure about this girl upon meeting her.  Also, that he called her demeaning names, despite he himself having been involved with her.  Finally, when I tried to explain why I was sort of upset by the way he told me this information, he couldn't understand my position and we left it at a sort of "agree to disagree" place.

Now, a month later, he is going out with his friends to a bar.  I asked him who was going, and he said his guy friends, and Rebecca.  I asked him if he invited her and he said that it was his friend who did.  Now I don't know how to feel.  I'm torn between being hurt by the fact that he is going out with this girl who he knows I'm not a big fan of, and trying not to be a jealous girlfriend because that, of course, is not what a trusting, full relationship is about.  What am I doing, Reddit?  Am I being completely overdramatic about this, or is it reasonable for me to be irked by this situation?

Could you distill the essence of the text into 1-3 sentences?
<bot>: Boyfriend is hanging out with a girl he hooked up with in the past and I don't know how I feel about it.