Tmi blood clotting please help

Tmi.    Leading up to my period I had sore breasts and pain in the lower belly and now I got my period and I have pain in lower abdomen and I just noticed a big clot when I used the bathroom.  It was dark dark red, black like and looked like jelly. Very think. I've never seen something like this before. Period has been regular.  Does anyone know what could cause this or what it is?   Thanks!!