What Causes Delay In Menstruation Other Than Pregnancy?

I had sex on 18th january 2014. and my due date of periods was 16th of feb. I took unwanted 72 within 3 hours of sex but i still haven't got my periods. That is I'm 20 days delayed from from my due date of my periods.
i've taken 2 pregnancy test and they have been negative, but i never miss my period i get it every month even if im stressed out.
Barnali