I Had A Pink Big Piece Of Tissue Come Out

I had a pink big piece of tissue come out of my vagina today. I shouldn't have my period because I started a new birth control 2 months ago taking pills orally everyday to stop my period because I have endometriosis and my period makes it worse. I got excruciating lower back pains and cramps over the past few days and have been bleeding and today a piece of pink tissue fairly big came out with the blood.