We had our girl put down at the start of last year. On NYE she was seemingly fine. Less than 2 week later she was skin and bones, hardly eating or drinking, her fur looked like she was an unkept, feral cat and she had no interest in doing anything other than laying down and she just wasn't herself. Had a distant look in her eyes all the time.
We took her to the vet who said there was nothing that could be done. She was full of cancer so a few days later we had her put down
It was horrible as it happened so fast and i didn't want her to go but i could see she was miserable and she wasn't herself anymore.
If she was still happy, eating, drinking, moving around i would have been fine to let her keep going.
Can you ring the vet and see what they say? Or like HR said can the vet come to you?
Hope you have your old girl for a bit longer yet. Good luck