My sex drive has decreased so much!

My sex drive has deceased like immensely! I use to want it any time of the day everyday now MAYBE once a week. Last night my fiancé tried to make a move but I quickly rejected him this is like the 2nd or 3rd time now that I've rejected him. I feel so horrible about it... I know that he wouldn't stray away I just wish I could give him the same energy and pleasure I use to. I'll be 12 weeks on Tuesday I'm hoping as this pregnancy progresses my sex drive will make a comeback.