The suggestion to talk to your MD is the best one I think. There are million different types of birth control, and different versions of each. If I was in your shoes, I would talk to the MD and ask him if you could try another type of IUD... cause there are different brands and different models... or if he thinks that based on your symptoms, you should try another type of birth control. My bestfriend got the Meridia IUD, and she stopped having a period three months ago. After two blood and urine prego tests of negative, her doc said that her IUD was known to do that to women. That one is, I believe, the size of your palm, and I am scheduled to get it inserted when I get my monthly flow, which should be soon!
They have other brands and models, so you could explore the different options. I also have a friend that is on the Neuva Ring... and she likes that. Again, talk to your MD... Good luck!