Im so jealous right now...What should I do??

Well, my boyfriend and I have been together for almost 4 months and according to him, he doesnt see any problems existing between us, too. And to be honest, Im not a jealous person, he went to prom with his ex and I didnt have a problem with it and I picked out the tux for him to wear…but there is only this one girl who always makes me worry because she would ask him to dance in front of me and they talk to each other online and in person A LOT recently and I sent him a really serious message and he showed it to her…actually I do have the intuition that there must be something going on but he didnt want to admit it =( I talked to him about it because I didnt feel good while watching them dancing together and its because I met him at a party and dancing brought us together so it’s kinda like our thing, it was really hard for me…and I didnt act immature or anything, I just told him how I felt and I didnt demand anything, but Im expecting him to show some respect at least, back off a little bit, but he didnt he danced with her again tonight *(well we had two parties consecutively lol_) and which really pissed me off…because she was flirting with one of my friends like a month ago and his girlfriend got SUPER jealous, worse than I did, so my friend backed off, I mean he has been avoiding her…and Im feeling like crap why can my boyfriend do the same thing…What should I do…I dont want to yell at him or anything, should we talk about it and have a long conversation but I already did…what else should I say?