Change of character in Mum. Any advice?

Not sure this is in the right category, but here goes. Mum has been in Supported Housing for almost 2 years now. She has her own apartment and carers who are based on site come in 4 times a day to fix her meals, etc. She's always had the odd grumble about the carers that come in, but lately she's getting very critical of them and has been bad-mouthing them in front of other the residents. Today the care manager has phoned me to ask that I speak to mum as they wont put up with the way she's speaking to them and the aggressive/abusive phone messages she's leaving on their office phone. This is hot on the heels of a call I've received from one of mum's friends (of 35 years). The woman was in tears because mum has been leaving her horrid messages too. Mums never been one to put up with other people's failings, but this does seem out of character. I'm wondering if it's the start of a UTI? Any advice.