I am sorry your cay has changed her toileting behaviour. there are many reasons for this change and it can be simply from a change in routine such as different cat litter being used, or a change in the litter tray position.
Often it is more associated with deterioration in cognitive function, that is the awareness of their normal routine, this is common in older cats, but hard to diagnose.
It is important that your cat is producing normal faeces, as diarrhoea would give a sense of urgency to toileting and often they can be 'caught short'.
Hyperthyroidism is common in older cats and can cause diarrhoea. If your cat has a normal stool, this altered behaviour may be nothing to do with the hyperthyroidism.
If she is an older cat, suddenly stopping using the litter tray can be seen if the legs are sore and it is hard to step into the tray.
SO, as you can see there are several problems that can cause this. So the first thing is to determine if the faeces are normal, if not and they are looser then it maybe an upset tummy that is causing this so a trip to the vets would be needed. If the faeces are normal, then watch her to see if she has changed her normal movements and is not struggling with steps or jumping, if she is then it might ab because of sore legs so again, a vet exam would help identify this and allow provision of pain meds.
Hope this makes sense
Dr Dan Makin