What Causes Delay In Periods Inspite Of Taking I Pill?

m 21 yr old girl n have unprotected sex with my bf on 28 jan but after 2 hrs i take the i pill to prevent pregnancy. i had my periods on 4th feb but very brown dischrge in very small qty that i even nt use any pad.before sex many times i got this type of spotting and having the problem of irregular and delayed periods.but this time i m very tensed because i had sex my pariod are not come and today is 24th march...what can i do?i know i had done wrong act and i m very sry for that act ...and about 2 weeks ago i also tried prega news to test pregnancy and the test is negetive but because of my delayed periods i m very tensed..m i pregnent??plz help me i can,t discuss this problem with any one......