IF you had unprotected intercourse
then pregnancy could be a possibility. BUT as u say your pregnancy tests have been negative, the other possibility could be just delayed periods.
If its the first time you have delayed periods, it could be due to change of environment, anxiety , stress, recent changes in your body weight( weight gain or loss). The other causes can be pathological like ovarian cysts, genital infections and Polycystic ovarian syndrome
You should first take up an ultrasound
to rule out pathological causes of delayed periods. If ultrasound is normal, then you can use progesterone tablets
for withdrawal bleeding. The dose and method of intake would be told to you by your gynecologist. Self medication
should not be done without any evaluation by a doctor.