Sorry to hear your cat is not well We lost our old girl in Feb at age 18.

With Boo she just suddenly looked old. She was always very vain and kept herself nice but she just looked very unkempt and frail, was literally in the space of a day or two. We knew it was the end, just a feeling we had, you know a pet when you've had them that long of course. She also hid under the bed and when we coaxed her out she didn't want to be touched. I wrote a thread on her at the time but we had to make that hard decision to let her go a few days following.