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Sudden onset deterioration in an otherwise stable person may be an acute infection such as chest infection or urinary tract infection. I would ring NHS24/NHS Direct for an appointment with an out of hours doctor to ensure that she does not need immediate treatment. Other possible causes can be side effect from her medication eg furosemide or painkillers/antidepressants
If there is no infection, and she seems otherwise healthy then she will need a longer assessment to establish the state of the confusion and she will need blood tests to look for vitamin B12 or thiamine deficiency, metabolic disturbance and electrolyte disturbance.
I appreciate that this is worrying, however I have seen patients with this pattern respond very well to treatment so do get her checked over.