Thank you for sharing your question with the EmpowHER community.
The ECP (emergency contraceptive pill) Postinor instructs to take it with 72 hours of unprotected sex, which is what you did, so it is likely that it has done it's job in preventing pregnancy. As for your bleeding, it's probably withdrawel bleeding as a result of the high dosage of hormones that are in the ECP. The high dosage of hormones in ECP will disrupt your normal hormone pattern and cause menstrual abnormalities.
I am not a doctor, but I hope this information helps.