Why is my boyfriend acting weird?

So my boyfriend is acting really weird. He doesn't talk to me much, even when we're together in the same room. He never texts me back until hours later. We haven't had sex for a while now, which is really weird for us. I try to do stuff, but he always says he's too tired or just not in the mood. And he never lets me look at his phone. If I pick it up he'll grab it away real quick and if he's on it and I glance over at it, he turns it off and he won't tell me the password. I just feel like he's being really distant.. and am I crazy to think he maybe doesn't want to be with me anymore or that he might be cheating on me or something? He doesn't seem to trust me and he literally never wants to have sex, which is so unlike him, so that's why I'm kind of worrying..