Hi,
We had a cat with this about 15 years ago. Our cat sitter was one of the veterinary nurses at the surgery - so no consultation fees when she was cat-sitting!
One holiday, she'd taken Coco to the vet who had given her an antibiotic injection which perked her up. She became much healthier, but her taste in food changed - she started liking peppers, and really strong cheese.
Some months later, we went away for a weekend, and came back to a note suggesting we took her to the vet from our cat-sitter.
Her general behaviour changed in that dh stopped being her human, and she wanted to sit near me, as she had done when a kitten. Before that she was a complete tart - arching her back and curling her tail over when dh brushed her. The harder he brushed her, the more she liked it. That was the only real change in her behaviour for the last, say, 3 months.
Sadly that trip to the vet was her last one. She was a beautiful long-haired brown/cream tabby with green eyes, looked just like a Maine Coon. We celebrated her 9 years with us with a bottle of champagne 
I think from memory, that she lived for about 3-6 months after the diagnosis. We vowed not to get another cat.
4 months later, 16 1/2 years ago, we came home with a small bundle of fluff. A brown/cream tabby Maine Coon kitten, who looks rather like Coco did, and who has been vaccinated against everything going since a kitten.
I hope it doesn't mean that it's the end but it's always worthwhile taking your cat to the vet, just to check that they aren't in any pain.
Take care
tb
xxxxxx