is possible only if unprotected sex is done around ovulation time.
You took NEXT CHOICE
emergency pill within 12hrs of unprotected sex. You have to take also second tab 12hrs apart. Then it gives more than 90% efficacy.
Now you got brown vaginal discharge
and period began. It is probably due to withdrawal bleeding of the pill. So don't be much worried. Also consult with your doctor regarding irregular period and need some investigation to detect underlying pathology.
Be well and practice safe sex