I know a few people have responded to this but I just thought I'd add my
$.02 worth.  My ferret Hobie also went through a period where he was
grinding his teeth.  At times when he would be eating or drinking he would
also shake his head.  We found out (after visiting the vets) that he had a
bad tooth which was promptly removed.  It could also indicate infection in
the gums or a cracked or broken tooth.
[Posted in FML issue 1854]