Why am i no longer interested in ? Please Help!!!!?

me and my boyf av been together for 4 years, for the first two years we had all the time and it was amazin, but the past 2 years i have completely lost interest, i still find my boyf attractive and when we do hae i still really love it but im hardly ever in the mood, what can be causin it? i have been on the pill for 3 years, and i smoke a lot of weed, i have lost a lil weight since i was first with my boyf(was 10.5 stone now 9.3) but worry about my body more than i use to! its really startin to piss my boyf off cuz i keep sayin no when he tries! and i dont know why