Hello,
I also had a similar experience, except the way I found out was by becoming pregnant (with my third)- apparently the IUD had detached without my knowing. Thats why they have you check placement, and it is also likely to happen within the first few weeks of placement, they should be able to just put a new one in. The other thing, is that apparently if you have a heart shaped uterus (they would know by the ultrasound) it is more difficult to hold onto an IUD. (Thats what I have so I had to find an alternative route to the IUD, which makes me very sad because I loved the IUD as birth control).
Its good that you checked for placement (they neglected to mention that to me) and it definately can happen, and I'd just talk to the doctor about their recommendations. If it were me I'd stick to it with the IUD unless there is a physical reason why you shouldn't but thats just me. You do have to trust your birth control! :)
Good Luck
E. (mom of three boys)