Let me say upfront that I am completely biased against the Mirena IUD. I had a large fibroid before the IUD was inserted, and I asked for one without hormones, and was told they are not available in the United States, so the Mirena was prescribed instead. I had gradual weight gain over 2 years, but no pain or discomfort - but my stomach got HUGE. I started getting nervous about the IUD, thinking I had a tumor or something. I went in to get it checked out, and the first thing the nurse did was give me a pregnancy test, b/c I looked pregnant. They did an external and internal exam, and finally an ultrasound. I had a fibroid tumor (not cancer, thank goodness) that was the size of an 18-week fetus!! Fibroids are known to be "fed" by hormone treatments, and I should never have been prescribed an IUD with hormones. I wound up having an emergency complete hysterectomy, with a huge incision, b/c my uterus was so big. I am angry that this happened, and that it was a direct result of the hormones in the Mirena IUD. I am also sad, because even though I am "done" having children, I am somehow hurt to lose the cradle where I carried both of my children for 9 months each.
As far as advice for you is concerned, you should not live with chronic pain associated with a medical device, and you should not have to rely on prescription pain meds. At your young age, you should not risk losing your fertility like I lost mine. Go to a different doctor and get a second opinion. Even if the Mirena IUD is not the problem, this condition seems to be and should be resolved if possible. Good luck, we'll be praying for you.