If it was after anal sex, then yoiu should probably see a doctor as that could be fecal matter mixed with blood. If this discharge was from the vagina, than it could be 'old retained blood' which is brown in color. Anal sex can be dangerous and was not intended as a way of having sex, so you must be very careful if this is what you are participating in.
that happens to mee at times when i have sex ,i dont think its anything to worry about
If this continues, you may want to be checked for an infection,if your boyfriend fingered you after anal sex, it sounds possible that anal secretions may have entered the vagina, thats why we wipe front to back, its easy to get a vaginal infection from fecal matter no matter how microscopic,
It may just be something called "spotting" which happens to some women between periods. It presents as a very light flow of blood which appears darker than regular period blood. But if you have regular periods usually, with no spotting, play it safe and take a pregnancy test.