Is Large Piece Of Skin During Periods While On Birth Control Concerning?

I have been having issues with birth control pills over the last 3 years (specifically, I was getting break though bleeding which turned into getting my period twice a month). I went off the pill for about a year, and recently went back on it (with a steady hormonal pill rather than one which increases hormones).
I went back on the pill about a month ago, and have been spotting for the past week or so (dark brown spotting). About 3 days ago, my period began. Today, when I pulled my tampon out, there was a large piece of skin attached to it. It looked almost like a balloon (it looked like a sack of skin, but it was empty. It was about the size of my pinky finger). I am a little worried. What might it be?