why am I not interested in sex anymore? p

Weird question but it's concerning me. I'm 18 and have been with the same partner for years, and we used to really enjoy sex. It's nothing to do with our relationship but me being the female sex seems uncomfortable for me now for no reason. At first it burns a lot and just feels uncomfortable but I also seem to get a lot of cramps, regardless of the time of my cycle. I also feel like my sex drive is very low for my age, it definetely didn't used to be this way. I am on the pill and have been for 3 years if that would have any affect, I haven't had any problems till a few months ago. It's really depressing me because I want to want to have sex and enjoy it with my boyfriend but I never want it anymore. Thoughts, ideas, concerns????