I have many symptoms that have come up or gotten worse in the almost 4 years that I've had the Mirena, but the worst is the chronic pain. Mine feels similar to a muscle or nerve disease, but my doctor has ruled most things out. It started as a shooting pain in my right arm almost 2 years ago, and now it's all over my body. Lots of aches and pains in muscles, bones and joints. I feel very stiff, like I would expect to when I'm 80 years old!
I just had my Mirena taken out a few days ago, and I will certainly be here to post as things get better! My dr. said it may take a couple of months for the hormone from Mirena to clear out of my body. I haven't had my period yet, so maybe that will help to flush things out.
Oh, and to answer your question, one of my side effects was also heart flutters, and that has gone away.