I've gone off sex completely and feel really bad for my fiance, what should I do?

My fiance is a lovely man. He's understanding and very patient. I have totally gone off sex and anything to do with sex. I don't know why this happens to me. I know I am very stressed over exams coming up and I am very tired all the time. the thing is i don't ever think of sex at all! ever! It's terrible. I get touchy when he is looking for it and I feel so bad but it's a huge mental and physical effort for me and on valentines day I broke down crying when we were in the middle of it and had to say sorry a million time. I just can't stop beating myself up even though he doesn't do it to me :(