How Effective Is Postinor As Contraceptive Pill?

Hi, please help me as i am worried sick. I had unprotected sex on 20th of December, i took Postinor 2 approximately 12 hours after sex which is on 21th. Now is the 1st of January 2015 and i still havent got my period. It has been 12 days since the sex. I have been doing pregnancy test for 3 times & all the results came in negative. I am worried if im pregnant. When will i get my period? Since 4-5 days ago, i started to feel the usual pms i always had whenever i was about to get my period, tender breast and cramping. oh and i forgot when is the last time i got my period as i have never paid attention to my period due. I have no idea when is my last period. Help?