Serious spotting. Ideas why?

I am on the Nuvaring and have been for months. My period always comes 3 days after I take it out without fail. I've never had spotting in my life and this stuff is getting really annoying. My period isn't due for another 13 days and I've been spotting all day everyday for the past week. I'm not over weight, I take vitamins, eat really healthy, no stress. My doc put me on a headache medicine that "thins" the blood, but I stopped taking it when the spotting wouldn't go away. Any ideas why?