I have pcos and rarely start my period on my own. i've been having very light (brown) discharge for about 2 weeks & bad back pain. what could this be?

Culture needed: You need to see PCP or gynecologist . The discharge needs to be cultured. But bec of your PCOS this may be your cycle. Hard to tell but infection needs to be ruled out before just saying this is your cycle.

Cyst rupture: These symptoms are reflecting complications of your condition. It will require blood evaluation and pelvic examination to determine definite cause of the pain. Rupture of the ovarian cyst could be one of the reasons for the back pain.

Back Pain: Go see your family physician to have your symptoms evaluated. The brown discharge and severe back pain could be a sign of something that needs attention.

Back pain, vag disch: Many possibilities. You need a medical exam to see about the PCOS and determine if you have some other problem e.g. a pelvic infection.