Hi and thanks for this query.
Having sex right after your menses, taking an emergency contraception
immediately after make pregnancy
an almost impossibility. You may need to look for other causes of this brown vaginal discahrge to make sure it does not represent an infection.
Please, seek medical attention because this isn't pregnancy and very important if it should persist. Also, always consider having protected sex. It prevents unwanted pregnancies and STDs.
Wish you good health.