Patient: How does Postinor 2 affect hormones? My mesntrual cycle came a week early on the 9th lasted 2 days heavy flow. Had unprotected sex on the 15th took a dosage of postinor 2 on the 16th. Menstrual cycle started again on the 19th lasted a day very light flow. Started again on the 23rd heavy flow and is still here today but very light.