What Causes Delayed Periods?

I had sex with my bf on 17-18th Jan. Then took ipill an emergency contraceptive medicine. I bled on 22-25 as side affects of the medicine. I was supposed to have my periods on 5th Feb but its been 2 weeks and I have not had my periods. I checked on pregnancy test kit and it was negative. What should I do. I skipped my period. Is it possible??