Hi, welcome to Psych Central.
Have you noticed any other changes in her memory or personality? Has she become more forgetful, does she know who her family are?
It could be many things, dehydration can cause a type of delirium, she could have a type of dementia developing or depression. Please don't be afraid to discuss this with a health professional. If there is something going on, then treatment can be started.
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Pegasus
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“Everybody is a genius. But if you judge a fish by it's ability to climb a tree, it will live it's whole life believing that it is stupid.” - Albert Einstein