What Causes Delayed Periods After Taking Postinor?

Hi I Am 27yrs had my period on the 11th April had unprotected sex on 15th. I took postinor2 within 12 hrs and the second dosage as recommended, also I took migraine medicine around the same time that I took the 1st pill. I haven't had my period since and I took several pregnancy tests which all showed negative... Plz help any suggestions?