I'm so disappointed!

So my s/o and I went over to his brothers house to watch the Pacquiao Fight and of course he invited people over.  Little did I know one of the girls he invited my s/o had sex with like 8 years ago, it bothered me because I met her in one of the birthday parties we went to and I had to find out from his brothers wife because she thought I knew what happened.  I didn't make a scene, I left the party I was upset because it was uncomfortable!  I was befriending the bitch and had no idea.  I felt embarrassed and I really expected that my s/o would have the balls to tell me before shit like this happened!  Then on top of it he thinks that there was nothing wrong with it, it's in the "PAST"!  When is it ever comfortable to hang out with someone your s/o had sex with??? And why is she still in the picture?!  Am I over reacting?! Because this really bothers me!