What Causes Delay In Periods?

hiii doctor dis is apoorva my age 19 weight 57 height 5.2 i dont have any health problem from bangalore last month i gone for the trip with my boyfriend my boyfriend had unprocted sex on my periods of 8th day every month my periods on 18th but dis month i did not get my periods only till now i checked in pergency kit it shows negative but i m too afaraid now plz help me now doctor what i will be pergent are not plz help me out