Typical subd prob by the sound of it so dont worry.

First off Convert it back to a poly then make sure All your verts are merged. And all history is deleted.

Convert to a sub d. Then in the hypergraph check you dont have any subd Blind data nodes. these are the Subd version of History. Simply delete them.

If the extra mesh is still showing you can also pick it out in the hypergraph and delete it.

you should now be able to switch back and forth between Subd and poly without any hassles and also be able to right click on the subd to show a proxy cage.

Jay