I think my ex likes one of my friends... should I leave it or cry about it?

One of my closest friends is your typical gorgeous girl that every guy loves. My ex doesn't really have any female friends aside from us. We dated over a year ago but I still have a few leftover feelings (he was my first). He always goes back and forth... flirting with her, then me, etc. I get jealous really easily so I usually care when he flirts with her. But lately I've just been like.. meh whatever I'm hurt but what am I supposed to do about it. And I would never date him again but I don't feel comfortable with him flirting with her and following her around. I don;'t think she would ever date him because she doesn't like him like that and she know show I feel. But She unintentionally flirts back... she's the type that flirts but doesn;'t know it. Should I just leave it be?