Since it started to pull the brown liquid of undigested cat food, I think it should be indigestion. Because the digestive system of cats of this age is already very bad, you can consider softening cat food with warm water, not too much at a time. Deal with the unfinished cat food in time. It is not recommended that you choose moist and smelly food such as Miao Xianbao, which is easy to accumulate gas in the stomach and cause indigestion. It is recommended to give warm water instead of cold water. If you can afford it, go to the pet hospital for a detailed examination. Intravenous drip can be considered, but it is estimated that it is also supplemented with some vitamins and the like, just delaying its life. It may cost more money, but it's worth raising it for so many years to tell the truth, at least I feel at ease. As for the location, because I am from Beijing, I don't know which pet hospitals are available in Shanghai. I believe that there should be more and more authoritative pet hospitals in Shanghai than in Beijing. You can check it online.
I hope to adopt.