What Causes Blood Clots In Menstrual Flow?

Hi I have irregular period cycle. I will have menses for 3 weeks. Then no menses for 2 then back to menses the following 3 weeks. I have had my menses now going on 3 weeks so I am almost done. The last few days I have had clots ranging from differ sizes..it's normal for me because of the heavy periods. Today I had a cramp then shortly after felt a clot about to come out. Sorry for the details..it had a lizard shape to it...the center was firm and black and the outter edges red but the shape freaked me out.