What exactly does brown vaginal discharge mean?

Sounds normal! During periods, we shed the inner lining of the uterus -- the endometrium -- if we're not pregnant. Sometimes that shed layer may not come out for a day or two & can turn brown. So can be normal; why it changed, i can't say. If you are having low pelvic pain, the discharge is unusually foul smelling, or your have a fever, though, it could be an infection -- then see an ob/gyn! good luck!