No more sex?

My boyfriend and I have been together for 1-1/2 year and throughout we would have sex regularly, and it was always fun and spur of the moment. And now recently he's not wanted to have sex. I tell him directly and even try to instigate it but nothing! And to me it makes me think crazy (like he's cheating, or cause I have gained weight!) and I've asked he said he's just not in the mood! But when I text him he says that's he's rubbing one out! Am I just parinoid or should I worry!