Why The Menstrual Periods Are Missing?

I AM 19 YEARS OLD. AFTER HAVING SEX I TOOK 1 I-PILL WITHIN ONE HOUR. FIRST 2 MONTHS I HAD NORMAL MP. BUT NOW IN THIS MONTH IT HAS NOT YET ARRIVED, THOUGH 4 DAYS ARE OVER FROM THE ACTUAL TIMING. IS THERE ANY PROBLEM OR IT IS A NORMAL SYMPTOM OF HAVING I-PILL.