What's causing my grandmother's rapid progression? Could she have something other than Alzheimer's?

My grandmother was diagnosed with Alzheimer's about 10 years ago. She's been at between stage 2 and 3 up until mid last year. Between just April and July of 2008, she seems to have progressed up stage 5 at least. In April, she could tell where she was, she had no delusions, and she could remember the names of all her family members. We were even able to correct her bad posture.

After about July of 2008, she kept asking where she was when she was at home. She had delusions of strangers living in her house. She forgot my cousin's name and his relation to her but she still recognized him as a family member. Her memory also worsened considerably. In April, she could remember at least half of what we tell her for a few hours. In July, she forgets 95% of what we tell her in a matter of minutes. However, her personality has remained fairly stable, albeit more exaggerated. Before she was diagnosed with Alzheimer's, she had problems with depression and she already had a very paranoid personality, bordering on a paranoid personality disorder.

What caused this rapid increase in progression? Is it possible that she's being affected by something else?