Low sex drive ?? Help!

I am 26 years old. My boyfriend  and I have been together for 6 years. I still feel crazy in love and I am always excited about the time we spend together. We have always had ALOT Of sex, like mostly every day. I usually always have an orgasim and we have always had a strong connection.  But, recently like the past few months my sex drive has slowly been decreasing. The past couple weeks I just simply don't want to have sex AT ALL. Usually my bf can get me back in the mood, but even when I try it's just terriable. I feel so weird and I don't enjoy it, it's kind if painful (even with lube) and he can tell something is wrong. I feel bad because he half way jokingly says "your not attracted to me".  He keeps asking what's wrong and I don't know how to answer! Ughh... I worry that it is starting to impact our relationship. And now with it being Vday, I know he expects something, but I'm dreading it and I feel so nervouse. Any advice? Is this normal? I mean maybe because I'm getting older? Or I feel a little sad in the winter??