I spotted (brown) for 18 days. i have pcos and rarely spot or start my period on my own. what could this be?

Endometrium: This is probably just the sloughing off of the thin endometrium that you have. Do you have any signs of an infection - vaginal itching, burning? Talk to your GYN doctor if this continues.

Breakthrough bleed: It's likely sloughing off of the endometrium that you build up when you don't have your periods-breakthrough bleeding or you may have ovulated and then had spotting. In either case, you should see you gynecologist for a check up.