NO SEX!?

My boyfriend and I have been in a relationship for 4 years and have always had a really amazing sex life, lately (last 3 months) I've not been interested in him at all, I'm still deeply in love with him and find him very sexy, but I just don't want to have sex, I still masterbate and all, but it's not fair to him that i just stopped having sex, any tips on how to get turned on and want to have sex? We both need help!