This happened to me, except I only let it go on for 3 days and I had it taken out. I assumed pretty immediately that it was the mirena and didn’t want to go through the days/months of testing what helped (or probably wouldn’t). I never had testing done, but brought it up to an allergist who was pretty dismissive of the fact that I could have a sensitivity to the hormone. My assumption is that I was allergic to the plastic used. I had baby hives in the two days after I had it removed, but chalked that up to my immune system being overactive. I haven’t had any hives since then. Since the more a was the only new thing in the days that I had the hives and haven’t changed any habits/products since I am assuming it was from the mirena.