Yikes! Have they done an ultrasound to see if the IUD has fallen out or otherwise made it's way out of your uterus?
I was having the same symptoms many months in a row (but never even a faint positive on a test) and it turns out my IUD was coming out on it's own. It just slowly made it's way down until I actually felt it poking me and TMI-could reach the hard plastic!! I had an u/s because of ovary pain I was having and they told me the IUD was down way too far, a few weeks previous to that, so I wasn't totally surprised when it happened, but it was still unnerving.
I hope everything goes ok and you get the answers you want and don't have any complications with anything. I'll be thinking of you!