What Causes Passing Blood Clots During Periods?

I am a 46 year old female with two children and when I was 24 I was diagnosed with a moderate case of endometriosis. I was treated with Lupron Depot an injection to prevent infertility. I didn't have any problems conceiving, but recently I have been passing blood clots the size of the palm of my hand during my period. Should I be concerned? I am 5' 5" and weight about 160 pounds