Overly-sensitive?

I’ve been dating a guy for the past 6 months and we were close friends for 4 years before that. A year or so ago (before we ever started dating) I tried to set him up with a friend of mine. They met once (not on a date) when I introduced them, he told me that he thought she was really attractive but nothing ever really came of it because he thought she was interested in another guy… I assured him she was not but he never pursued her or asked about her after that. I think he thought maybe she was out of his league or something.. ?

We started dating 6 months ago and everything has been great. Recently he came over to my apartment though on a night we were going out – my friend that I had tried to set him up with a year ago was over with her new boyfriend (she’s also friends with my roommate). The guy I’m dating acted totally different around her and was (what seemed like to me) flirting with her a bit. Ever since that night he’s brought her up several times, asking if the guy she was with was her boyfriend and also saying how beautiful she is. It’s really been bothering me – I’ve tried to just forget about it, as I’m not typically a jealous person. Seeing the way he acted around her though made me wonder if he ever was really that interested in me? I feel that he would really rather be with her but is with me because I’m available and she’s not- I feel like the connection has lost it’s charm a bit for me. Part of me wants to end things – But I can’t tell if I’m being overly-sensitive?