What Causes A Dark Brown Discharge In Menstruation?

I'm on the Nuvaring and have been for over a year. My last period was May 21-24th and I'm supposed to start this coming sunday, June 12th. I've had a dark brown discharge in my underwear that started June 7th and is still showing up. I did have sex on May 25th but wasn't supposed to put in my new Nuvaring until the 26th. We used a condom but could I still be pregnant or is this dark brown discharge normal?