boyfriend wont have sex with me?

He's home with me ALMOST every night. Recently he started a new job, and sometimes he's had to stay an hour or two past time to finish up work. It's been almost three weeks since we've had sex, and he usually says it's because he has to get up early, or that he's too tired. But in the past he's always wanted to.
I literally threw myself at him today, saying when you get home from work I'll be waiting etc. and he shot back with a maybe if I don't stay late here.

I don't know what the problem is, I feel so unwanted.