Another question

So my boyfriend and I have been together for now three years. He's absolutely wonderful. Takes carw of me in every sense of the word. But lately our sex life has been disappearing. I don't know what it is but I haven't been in the mood and I don't mind it. I feel terrible cause I know he really wants to and I just say no not right now and so on. I don't want to fizzle out it's just ugh