What Is The Reason For Delay In Periods?

I took Postinor within 3 hours after sex. At that time I was still having my period. It was the third day (19th June). I had my period on the 17th. Days later, on the 23-24, I started to bleed and it was like having another round of period. It lasted about 3 days. But now, my period is not here yet. Should I be expecting it on the date it bled again 23-24, or my period should be coming as usual, that is the 17th?