What Causes Delayed Period Post Medical Abortion?

Hi I take 4 tablets of nordette and after 6hrs I take another 4 tablets. After 4 days of taking it, I experienced heavy menstruation start from january 21-27,2015 which last until 7 days. But until now I still don t have a menstruation and I already took a pregnancy test twice but the result is negative. Is it normal that I still don t have my menstruation even though i only take nordette for 1 day only please help me. Thankyou