I had sex for my first time a week ago now I been having brown stuff in my underwear for about 5 days now. What is it?

Old blood/discharge. The first time you have intercourse often causes minor injury to the hymen, a delicate membrane at the entrance of the vagina, Minor injury to the cervix may also happen, especially in younger women. The resulting blood combined with vaginal secretions and the guy's cum can cause a discharge like this. You're very young. Please protect yourself!!!!!!