I found this article which was very interesting. I think it's a fairly new idea, but they have found that people can have allergies to progesterone and estrogen. They think that premenstrual asthma, menstrual migraines, interstitial cystitis and fibromyalgia may be related. People who had these things before Mirena might especially benefit, but I bet there's a connection with Mirena users as well. Dr. Roby out of Austin, Texas knows a lot about this and is mentioned in the article. It was also very interesting reading his website.
http://www.sciencedaily.com/releases/2006/03/060330182210.htm