I wonder if a specilaist may be in order. Do you have a lot of confidence in the GP?
Is your grandmother on any other meds or drinking alcohol? Has she added any new meds?
Im not a doctor but we just did Alzheimers in my MA program and were told that onset in your 90's is unusual...and to check the medications first.
In any case Im sorry about yor grandmother. It is difficult to see someone you care about deteriorate like that.