What Causes A Delayed Period Despite Normal Bleeding In The Previous Month?

Last month i had withdrawal bleeding for 7 days...dont really know whether its the withdrawal bleeding or normal periods...after taking unwanted 72..last month...the bleeding last month had started 7 days after taking the pill...but this month..one month later its already 1 day late since the day my periods started last month but there is no period...why?...plz reply soon its urgent