What Causes Delayed Periods In Spite Of Taking I Pill?

HI Dr. How are you?Actually i had unprotected sex with my gf on 14th aug,after spending golden time with her i gave her i-pill within 1 hr.After 1 week on 24th aug she got her periods Acually her manstrual cycle date is around 23 or 24 of the month but today is 26 and still she has not get her periodsand there is no sign of periods we are very tense pls help us there is any chance of pragnancy we had already chek with Urine test and Urine test showing nagative result(showing only single line)pls tell us that she is pragnant or not pls help us we are very tense.....