There are many urticarias, and they are related to dysfunction of the immune system. I had exercise urticaria (mild) for a little while last year but it went away after a couple months. I do know, by experience and from what I've learned on this forum, that the Mirena can mess with the immune system big time. We've all had our health ruined by the Mirena, and allergic reactions are just one of the many issues women have developed. I would suggest you have it out ASAP. Many of us had odd symptoms that came on gradually with the IUD, which we did not link to the IUD at first. For me, they started building up, one at a time, until they culminated in a massive neurological illness that made me feel I was dying. I started researching madly and came across the Mirena link and then had the IUD removed a month.
I don't believe anyone on this forum started having new health symptoms with the IUD and then ended up improving.
I would give anything to go back in time and tell myself to have it taken out at the beginning when the odd symptoms first started. OK, actually I'd go back in time and myself to NEVER have it placed at all!