Hi,
Welcome to ehealthforum,
The brown bleeding could be suggestive of pregnancy (due to history of intercourse during probable fertile period). Other possibilities include it being due to old blood coming, vaginal or cervical infection, cervical dysplasia, polyps, ovarian or uterine causes, use of hormonal birth control, vulvogvaginitis etc.
If pregnancy is suspected, look out for development of any other new symptoms suggestive of pregnancy. Wait and watch for periods, and visit your doctor/gynecologist if you do not get your periods or have altered periods. A thorough examination (to confirm pregnancy related changes/signs in body) and blood/serum HCG level test (to confirm/rule out pregnancy accurately) would help.
Take care.