Can Antibiotics For Pneumonia Cause Mentally Confused And Hallucination?

Hello, my mother, 81, was admitted to hospital with pneumonia 4 days ago; she was treated with antibiotics intravenously and seemed to be a bit better for 2 days, but yesterday she was extremely agitated and mentally confused, hallucinating, etc. I suspect she has a UTI, but last night there we no doctors available to treat her (Sunday night in the UK). In addition she is a cancer patient. Since I understand that antibiotics are used to treat a UTI, and she has received a massive dose of these the last few days to treat pneumonia, I am concerned. It is very distressing to see her in such a confused mental state. What should I expect to happen now, and what questions should I be asking when I visit?