is it wrong to be jealous? should I do something?

okay so there is this boy that ive been "with" for a little while (We aren't technically dating but we act like a couple, we've dated before and we've talked abt getting back together pretty soon. we have also said that everything we do us exclusive) hangs out with this other girl who he has been intimate with in the past. she has a boyfriend but is known for cheating and going behind people's backs. he knows that I don't really like her because of their history and that it bothers me, but I still see him with her or talking to her every now and then. i feel kinda bad for being so protective over him because him and I aren't officially a couple, is this stupid? Or should I bring it up again? and is asking him not to talk to her to extreme?