What causes vaginal brown discharge?

Blood. There could be several possibilities but the most likely is blood. You may have stared spotting but the flow is very light. Consequently the blood can become oxidized in the process and turn from red to brown. The blood could be associated with abnormal uterine bleeding or infection and may need to be evaluated if it persists.

Related Questions

What causes terrible smelling brown vaginal discharge for at least three days after each period?

Infection possible. Because of the foul odor, you should probably be seen to rule out infection. If you are a tampon user and leave it in for prolonged periods of time, this can also cause odor. Some slight odor around the period may be normal. Read more...
Sounds like BV . Sounds like bacterial vaginosis mixed with menstrual blood. Bv is an overgrowth of normal vaginal bacteria. The only real way to fix it is with antibiotics. There is an over the counter product called rephresh which can help normalize the ph of the vagina. Read more...