When a cat gets to be this old and has a large belly but is thin everywhere else, the chance are pretty good that she has a very serious issue with her liver. Most of the time it's liver cancer but it could just be that her liver is done. She is much older than most cats make it to, and you are to be commended for that, but she is just wearing out. I had two 19 yr old cats. One died of liver cancer and had the same symptoms as yours. The only one died of kidney failure one month after he turned 19. You need to get her into the vets and you also need to prepare yourself for the inevitable but very hard decision of what to do when he tells you this news.
We love these cats as our own children, and nothing can really 'prepare' you for losing them. But what you can do is resolve in your heart to not be selfish(which is what even I wanted to be) and let her go in peace when there is nothing more than can be done for her. By that I mean having her put down and not letting her suffer so she can die naturally. Yes, many times they DO suffer before they die.
I hate to be the bearer of maybe bad news but it is pretty clear that she is having some major issues.
I know this is hard, but you will do the right thing by her. She relies on your for her safety and comfort, and you will provide the gift to her of not letting her suffer.
Please let me know what the vet says. My prayers and thoughts will be with you both.