Why My Period Is Delayed?

hai doctor i had sex with my boy frnd apirl 14 but i didnt had heavy sex iam having fear about pregnance coz i had my last periods at apirl 5 till now it didnt came .i took pregnance test at home its result was negative then i went to hospital 2 dayslater to take pregnance test again its was also negative but till now i not have my periods may i know the reason .plllzzzz help me doctor am waiting for your reply