What Causes Missed Periods Even After Taking Unwanted 72?

hello doctor
i m too depressed. i don't know what exactly happened to me. and iam very scared of going to any gynecologist. i m 24yrs old women.and had unprotected sex with my fiance and i had taken unwanted72 within that 72hours period. but i have missed my periods. i brought pregnancy urine test kit it also showed negative but still i don't know why i haven't got my periods. i m worried please help me . i cant tell my parents and i m into last year of my degree. please madam.i dont wanna tell or take my finace along with me. i dont want to create any bad situation for my parents. please