What Causes Delay In Menstruation Other Than Pregnancy?

I had unprotected sex last week. I took the Next Choice pill an hour later and the 2nd pill exactly 12 hours after the first one. It was the day before i was suppose to start my period, and the 2nd pill the day of. my period is now 5 days late. i took a pregnancy test yesterday and it came out negative, but i still do not have my period. should i be worried that i am pregnant?