It's been couple of month and I haven't had my periods I know forsure I'm not pregnant.. So what can be the reason? I'm not married, and I'm only 21.

May I know what age you started you period? It happens. I once has 6months with no periods and I went to see GP and he said nothing to worry about, but I don't know about your case, do see your sexual health nurse at your surgery or give GP a call.