Our cat Jasmine is about 14 years old and has slowed down in the past couple of years, but has always eaten well and popped out into the garden to have a snoop around when Jasper isn't looking! If Jasper sees her she bats him off with her paw and basically tells him to get lost! The past 2 or 3 days I have noticed a gradual change in her. She is now hardly eating and can't even be bothered to bat Jasper away. She is pretty lethargic but doesn't seem to be in any pain and has no sickness that I have seen. Today she ate a small amount of chicken, but not that much. A few moments ago D picked her up for a cuddle and I noticed that her pupils are fixed and dilated
Her sister died a number of years ago from a brain tumour and her pupils went like this. I'll see how she is in the morning and then take her to the vet to give her the once over.