My grandmother did not recognize me today wtf?

I am almost 28 years old. I normally see my grandma about once a week to once a month depending on my schedule. I know she has Alzheimer's Disease and has had it for a few years now. However today she actually did not recognize me or know who I was. She is 83. Is this normal behavior for a person with Alzheimer's Disease? I have known the person for nearly 3 decades and she had not the foggiest idea who I was...