I've been having heavy brown discharge for five days now. my period is not due until another week. this has never happened before. what could this be?

Vaginal discharge: Are you sexually active? If so, may need to rule out stds. Do you have pelvic pain? If no, and not sexually active, and your discharge stops, wait another month, chances are you just had your period that came in early, which happens in females frequently. If in doubt, or in pain, see your doc, you may have a ruptured ovarian cyst or something else that mandates a check up.