He won't have sex with me

Me and my boyfriend have always had great sex! But the past 3 weeks he hasn't been interested at all, if i even try he just says "babe stop" or "I'm to tired" or "can we just lay down and watch the movie" like wtf?! And no he's not cheating trust me, i thought so to but no he's not i just don't know what it is, is it me? Like i have no idea what to do to make him have sex with me. Any advice ladies?