What Causes Missed Period After Taking Morning-after-pill Followed By Unprotected Sex?

I had unprotected sex, took the morning after pill within 12 hours and had a shorter than usual period about two weeks earlier than I should have. Now I've missed my next period which should have been the end of last month (February). I haven't taken a pregnancy test yet but I'm beginning to seriously worry. Is there anything you can tell me to help alleviate this?