I'm 15, had unprotected sex and my period is a month late

It is upsetting to feel used by a mate in this way. He was being deceptive and should have made it clear to you that he had a girlfriend, and that he saw your relationship as casual sex. The only consolation is that you made a mistake and will have learnt a hard lesson. If your period is a month late, you could be pregnant, and I would advise you to take a pregnancy test (read more info on pregnancy testing) or go to a pregnancy centre in your area to have a test done. They would also be able to support you if you have a decision to make.