I had the exact same symptoms. I've had the Mirena in for 5 weeks then started to feel off. Headaches, joint pain, sore neck and breast tenderness. I went back to the Dr who said it's absolutely unrelated and suggested I keep it in for 6 months. After doing research, I demanded to get this foreign object out of me. I got it out today and literally, my brain fog has gone away. With the Mirena I also had weight gain, no desire to workout (which I did before this), no desire to cook or bake (which I did before) or even leave the house. I went back to another doctor today who said my symptoms were hormonal.
When she pulled it out, she said showed it to me and said this was the devil that was the cause of your pain. Since having it out eight hours ago, my appetite and energy are back.
It works for some but I wasn't part of that. I'd rather have a sore back and cramps than to deal with this again. Good luck to those who have it.