I got my periods on 19 jan 2023 and had unprotected sex due to which i have to eat an oral contraceptive on 20th.today is 25th and my periods are not stopped. what can be the reason?

If the "oral contraceptive" was a one time dose ("Plan B"), withdrawal bleeding can occur in addition to your normal menstruation, as the estrogen wears off. However, since this can also mimic a pregnancy outside the uterus, (ectopic), obtain a serum HCG test to be certain about possible pregnancy, and see your GYN physician if the test is positive for other testing (ultrasound).