Light period with dark blood

I am on the birth control NuvaRing and when it was time to take it out I got my period but it's very light, I use tampons haven't filled one yet, has been 2 1/2 days.. blood is mostly dark, no fresh red blood. I am also VERY crampy. This is unusual, the only thing I can think of is pregnancy?? I didn't use protection the first week of insertion.. and twice removed the ring during intercourse and was left out for a maximum of 6 hours. By the way, I don't want to be pregnant, so any other ideas let me know.. Thanks