Personality changes from day to day?

My mother is in a rehab facility and will be going into an assisted living home soon. My question is this: I go see her one day and she is coherent, in a good mood, her voice is clear, she can feed herself somewhat, she is responsive. Then the next day she can be very different; unresponsive, raspy voice, incoherent, in a bad mood, very tired sounding, can't feed herself. I don't get this. She does have dementia. She is 92 years old. I don't understand. Is this common behavior???