What Causes Delay In Periods?

I had unprotected sex just before the day my period was due. My period didnt show up the due date so panicking I took an Ipill withing 48 hours. My periods show up for the next 3 days. Then I was prescribed Primolut N tablet. But my Period has still not arrived its one week late! Also i took 2 Home pregnancy test. and they both were negative. I took it 3 days after my missed period and 5 days the missed period.
What is the problem and normally my periods are pretty regular! 30 day cycle.