What Causes A Delayed Period With Negative Pregnancy Test?

Hi doctors i had unprotected sex with my bf last jan 22, i took 4 nordette 4days after the sex then after 12hrs i took 4 table again. My LMP was jan 13-18, then i had period again 1week after i took nordette (jan 28-30). Im expecting my period feb 13 but until now i haven't have my period. Im delayed for 11days i think??. I take PT but the result was negative. Im so worried.. what I'm gonna do?? Am I pregnant?