withdrawal bleeding or my real period?

Hi! Sept. 18 was supposed to be the start of my period. I also had unprotected sex(my first sexual intercourse) on the said date but my bf used withdrawal. In the past few months i have been experiencing late periods--ranging from 3-8 days. I took nordette ecp last sept. 20 and still i have no period. Now, sept. 26, i am already bleeding already with clots which seem to be that of my usual period. How do i know if this is already my period or if this is a withdrawal bleeding? And, is it possible that you are still pregnant after withdrawal bleeding?