Although I am not a Dr. I would imagine that what has is happening is that you are basically shedding all of the lining of your uterus that has built up in the past 10 months of not having a period. I don't think the clotting is abnormal at this point because it's most likely been in there long enough to clot. I would also expect it to be very dark in color and have a stong odor. Give it a week or so barring no other complications and if you haven't experienced at least a reduction in symptoms call your doc. I'm not a fan of Mirena but I really don't think, at this point, you have anything to worry about. Hope you feel better!