What Causes Heavy Vaginal Clots Post Miscarriage?

I had a miscarriage on March 5 of this year. A few days ago I began my first menstrual cycle post losing the pregnancy. Today I passed two walnut sized clots and have had more cramping than previous. I am 37 years old, this was my third pregnancy. I had a termination at 19 years of age, I have a 5 year old healthy girl that I had at 32 and this was my first miscarriage and it was twins. I was 8 weeks. I had a visit with my doctor post miscarriage and everything checked out. should I be concerned with these clots?