My girlfriend of over three years has really slowed down on sex. please help?

Now we HAD a great sex life, it had passion and excitement and she was always willing to get down and dirty. But about a year ago she really slowed down. she went off the pill, claiming it gave her headaches but when she was prescribed a better pill to stop it she never took them. She says its her not being comfortable with herself but she has lost weight and looks better than ever at about 48kgs. her overall attitude and affection have not altered only her sex drive. we still cuddle and such but if i try to turn it into foreplay she kinda laughs and pretty much immediately stops. I love her So much but it is driving me insane. particularly, because she talks with her friends about it like she's a regular in the sack. It is also leading to some pretty intense jealously streaks, normally im anything but jealous, but lately i want to fight everyone who hugs her. i have even started to rage over those kiddie photos where chicks pretend to make out for Facebook. I am kind of on the end of my tether and I'm afraid that if it doesn't stop I'm going to break up with her (i basically started my life with her)and do not want to do that. Please help i think since this started we have had sex about four times in the year and i honestly feel the tension building and do not want her or anybody else i care about to be on the receiving end of that tension when i snap