Pregnanct and not in the mood for sex

I used to enjoy sex so much, we used to do it about 3 times before going to bed and now I'm 17 weeks pregnant and I just don't want it. It even bugs me that my husband asks for it. 
Why did this happen and did it happen to any of you? 
Even when I'm doing it I just don't like how it feels.. I feel so bad?