Holy cow! I hear you. I'm 5"8 and keeping weight on has always been a struggle. I actually was looking forward to possibly gaining weight once I had Mirena inserted, but whether because of the IBS it caused, bouts of nausea and reflux, or generalized problems with absorption, I also lost weight on Mirena no matter what I ate, or how I altered my activities. I think I increased my calorie intake by at least 50% with no increase in weight. At my lowest point I was 106lbs, and could hardly eat because of nausea and reflux. I'm used to being 120-125lbs. I felt and looked so terrible I didn't want to go to work or out with friends, and tried to wear clothes that would hide my weight loss.
I'm 5 months post Mirena now and slowly starting to gain weight again. I'm still much skinnier than I'd like to be, but don't think I look so sick anymore. Plus-- my jeans fit again, yay!