What Is The Cause Of Delay In Period?

Hello doctor! i have a query related to my periods, i had an unprotected sex on 4th of july and within 4hrs i took ipill and usually my due date of period is 8th but my periods are delayed and today morning that is 11th july i started with my periods but blood colour is very light, can you please tell me the reason why does it happens as i dont want to be pregnant. plz tell me can i be pregnant?