If you go on the Mirena website, under physicians info, on page 5 it lists all kinds of side-effects that most doctors fail to discuss with patients before insertion. It mentions developing migraines as a reason for removal, so obviously it has been a problem for some people.
You might want to read that section as you may have had other weird things going on that you didn't realize were caused by Mirena. Otherwise, you can search on this forum as somebody has probably experienced what you are going through.
I am glad to hear that you are removing it soon. Best of luck to you !