What Causes Delayed Period And Vaginal Spotting While On ECP?

Hi, This is Nudrat here, My periods due date is 13th or 14th of every month. This month on 5th March I had unprotected sex with my boy friend. After that within 72 hours I took two ECP Pills. And according to my periods date it should be come on till 14th march. Periods come on 15th march in the form of spots and then stop bleeding. I am much worry about pregnancy. Please help me what should I do???