Hi,
Welcome to ehealthforum,
It is difficult to comment on the cause for the slight vaginal bleeding that you describe without examining you and going ahead with further evaluation done. The cause could be attributed to the underlying problems of PCOS and endometriosis if you are not on any form of treatment. If you are not on birth control, and have been sexually active, then the altered bleeding (brown discharge) could also suggest pregnancy possibility. Do not ignore your symptoms. Visit your doctor/gynecologist at the earliest for thorough evaluation to look for the underlying cause. Treatment as appropriate (based on examination findings and investigation results) would be started to help you control the symptoms. Be in regular follow-up and monitoring of your treating doctor/gynecologist, and report any new/abnormal symptoms immediately.
Take care.