Welcome to HCM.
Your LMP was 19th Aug and you did sex on 1st Sept just prior to ovulation or within fertile zone. You took postinor 2
immediately and second tab after 12 hrs.
As you end your period 2days ago,there is no chance of pregnancy
and Postinor 2 is quite effective contraceptive
method with success rate 98-99%.
You get brown discharge after period over due to old endometrial tissue and is quite normal.
So need not to be much worried about this and practice safe sex