TMI!! Tissue clots

So i've just started my period and i've had these tissue clots come out.

I've only ended up having this once before and they had originally thought i have had a miscarriage. They were unable to confirm it so i'm worried.

I had very painful cramps before passing these but i thought my flow was just heavy.

I did a pregnancy test today before my period started because it was late and it was negative.

Do you think this could be endometriosis clots?

Will be contacting my fertility specialist to get her opinion but she's out of office now.