Question about the Mirena IUD...please help?

I am getting my Mirena inserted next week. I had a baby 4 months ago and dont plan on another any time in the near future! After having my daughter I was given the dep shot which caused me to hold onto all my pregnancy weight making me 30lbs heavier than I was prior to getting pregnant. Now I cant loose the weight. My question is this: Will Mirena cause me to gain even more weight or make it hard t loose the weight I have now? Also does it hurt when inserted? Thanks for the help guys :)