What Causes Large Tissue Blood Clots Along With Abdominal Cramps During Menstruation?

I m 42 and on the pill to control periods. I have had a tuble many years ago. So I can t get pregnant. However, I m having bad cramps with my period. Blood is very red and I passed skin like substance with dark blood stinging from it. Now pain is gone only spotting now. What could this be?