Jealousy

So, I am a year into my relationship with my boyfriend. We met at a bar through a mutual friend, the two of them ended up hooking up that night, about a week later he and I hit it off. I knew of their one night stand from the begining, it was no secret. But now anytime he has any contact with her (only public things over Facebook, that I'm aware of) I find myself really jealous... I just don't get it. *sigh*