Dear Stephany,
You don't mention whether your mother was treated at a comprehensive cancer center or not. If you get a second opinion, which you definitely should (and a third opinion if necessary), make sure that you get these at a comprehensive cancer center. There is a listing of comprehensive cancer centeres elsewhere on the OCF web site.
Also, I don't understand why the doctors don't do a CT scan or some other imaging technology to see if there is an identifiable tumor that has returned. If they see something, they may be able to do a fine needle aspiration biopsy.
My thoughts and best wishes are with you and your mother. - Sheldon