I also had severe digestive problems with the Mirena. I lost far too much weight, was vitamin deficient, and after being treated for stomach infections over and over again (which made my digestion worse, not better), was tested for allergies and put on an extremely limited diet. Nothing really helped. It took a very long time from the onset of stomach trouble (summer of 2008) for me to realize it had to be the mirena. After removal in June 2009 my stomach felt better, but I was still having loose stools for a long time after.
It's a little over 18 months since removal, and I am finally gaining weight and have had pretty normal digestion for the past couple months.
Beware, post-removal, I had pretty horrific heartburn for a couple of months. However, things have gotten better. And while it's taken a long time for me to start to have normal digestion, I can say that even with the heartburn, the days that I am comfortable and free from nausea, and have a good appetite far outnumber the days I feel bad. Now there's probably only one or two days a month where I have stomach issues. I am still on a special diet, but if I stray I usually don't have issues.
Good luck, and get it removed ASAP! Recovery can take awhile, but you will eventually get there!