What Causes Delay In Periods After Use Contraceptive Pills?

I had unprotected sex with my partner on 20th of August and within 24 hrs i took ECP. Then after 7 days i had bleeding which last for 4-5 days. My periods were due on 12 september. But, as of now I did not experience any spotting. I did 2 pregnancy tests, but, they also came out to be -ve. I consulted a doctor on 1st october and she checked my blood pressure, breasts and even pressed repeatedly around my abdominal area to check if there is any kind of pain that i might feel. The blood pressure was perfectly normal as that of any normal woman, she said the breast were okay and there wasn't even any pain. So, she prescribed me to do a course of Regestrone (1-1-1) for three days. So, I kindly request you please tell what could be the probable problem as i am very tensed about it.