husband doesn't want to have sex.

My husband and I have been married for 3 months now. But for two months my husband can go weeks without being affectionate with me or want to have sex with me. When I try to initiate sex (like last night) he makes excuses and says he's not in the mood. My husband is 26 and I'm 22. But I don't think he should have such low sex drive at that age? I have gained about 8 lbs since we got married but I don't really think that could be it and definitely don't think he is cheating. I ask him if he's not attracted to me and of course he says he IS but that's hard to believe right now. We want children and don't use protection when we do have sex but I don't see us conceiving any time soon since we hardly even make love. Any advice ladies?