Not sure if this will help, just found it on another website. Call the doctor.
Brown vaginal discharge is most often associated with old endometrial tissues. In other words, if your period is late you might get a brown, rather than red, discharge. It could also mean that for whatever reason during your last menses the entire uterine lining failed to make a timely exit. What you are seeing now is the fashionably late exit.
In some cases however there is cause for concern. Brown vaginal discharge can also be a symptom of a medical concern that is much more sinister than leftover endometrial cells.
Some common conditions causing brown vaginal discharge include: cervical cancer, Pelvic Inflammatory Disease (PID), perimenopause, menopause, or sexually transmitted diseases like gonorrhea, Chlamydia or vaginal warts.
I think you need to go to a doctor and get it checked out. Everything I thought it might be you say it isn't so I really don't know what it could be.