You don't state your age ... it's possible it is peri-menopause starting up (which unfortunately can be years or even decades to go through). With menopause or peri-menopause, it is not uncommon to get what is called clotting. Or there may be some endometriosis.
Also, if there has been any internal trauma to the uterus, or any recent surgeries to the area (even if it was a month or two ago), you can get clotting.
Clotting can be scary, especially if it is new to you. First time I got clotting was about two months after a surgical procedure. I had several clumps come out about the size of the palm of my hand that looked like hunks of liver ... what they are actually is just congealed blood/tissue. I also got similar bad clotting one time when my bike wheel hit a curb and I flew head first over the bicycle, hitting my abdomen/uterus area with the twisted handlebar. I get smaller clots nowadays, but in larger quantities, and I'm sure it is due to just getting nearer that time of life.
Usually when I get clotting, the flow is much heavier, almost gushing.
But, when doing any detox (and Iodine supplementing does detox) sometimes the way a woman's body quickly dumps toxins is through the menses. So clotting and heavy flow that is not due to trauma or hormone changes may be the body ridding itself of toxins in as quick a manner as it can.