What Causes A Delayed Period After Taking I-pill?

Hi Dr.! i had unprotected intercourse on the 25-26th dec.... i had an ipill oth the 27thdec... my period was due on the 1st jan but still hasnt come..Today ie 4th jan i did preganacy test but it was -ve.... i was also taking meprate in oct for 10days 5mg twice a day>>> does it also effect the cycle?? plz help...