I Have Been On Birth Control For 5 Years, The

I have been on birth control for 5 years, the Nuvaring since last May. This past month, I left my ring out in the cold. I called my pharmacist and he said it was still okay, so I inserted it on time and have had intercourse a few times. I'm set to take my ring out in 3 days, but I've had heavy brown/black discharge with clots for the past 6 days. I've taken two pregnancy tests, both negative. What could this be?