I had a copper IUD, not mirena, and had an episode of excruciating pain. When I went to the hospital they said my IUD was fine (but they took it out anyway) and did some tests... it wasn't the IUD that caused my pain, it was a cyst on my ovary, probably it got twisted or something. So I would think your cyst is to blame here, do you know what kind? Maybe you have endometriosis like I do.
I am on the Nuva Ring as well and have no complaints
Everyone reacts differently to the many forms of birth control. I have a few friends that had the Mirena removed because of the exact problem you are having. If the pain is that excruciating and the bleeding is very bad then you might want to think of another form of birth control. The nuva ring is one of the few I haven't had any problem with, nor have I heard anyone complain about it.
The pain could be from your cyst on your ovary, my mom used to get those really bad at certain times and she said they were so painful she couldn't move.
Also my friend was on the mirena and somehow it got moved or something like that and she was bleeding really bad and in pain that she decided to take it out.