I think my boyfriend is cheating?

You're right.If I haven't seen my girlfriend for 3 weeks, yep, next time I see her, you bet ill be having a sex marathon. Even if I were tired, I would tell her, hey, let me sleep a bit, and then ill be back to normal; this has happened to me before; I used to work 12 to 16 hour shifts, and my girlfriend lived 5 hours away, so on weekends, I would go see her and as tired as iwas, we did a quickie, then I went to sleep, and woke up to start the real marathon. So that's why the excuse of I'm tired doesn't really convince me; also his response of , you didn't see me is quite terrible. Unless he was really tired, and having other types of problems, like a family member dying or being diagnosed with some mortal disease, then yes, maybe he couldn't perform; otherwise, I would be suspicious of him having booty on the side.

It doesn't look good just based off his answer to when you asked him if he was cheating on you. Though, it isn't definite. He could really be tired from work. I know that when a guy is too tired for sex, trust me, he's too tired, and he won't want to do it. Same with women I bet.

If he continues his ignoring and erratic behavior, I'd confront him about it. Don't accuse him of cheating just sternly ask him why he's been acting so differently.

I would say he's either cheating or just isn't attracted to you anymore. Unless you're long distance, no guy would want to be away from his girlfriend for three weeks. And if he was, he'd want to have sex immediately.