Miscarriage or tissue??

My birth control choice is Nuva Ring. At the end of three weeks, you take it out, you put another one in, you don't get your period. My nuva ring was in two days ago when I got my period, which never happened before. I took the nuva ring out, put a tampon in, whatever, okay. Well two mornings ago, I took my tampon out and on it was a huge chunk of flesh. It was bloody, gray, and was not a clot. It was flesh. Of course, I did not throw it away, I called my doctor and talked to the nurse and asked her about it and she said she never heard of this before. It very may well have been a miscarriage or just a piece of old piece of tissue. What could this be?