1. Withdrawal or sleeping too much. Sick cats in general withdraw in a corner of the room and are no longer interested to play or do all the other things they usually do. However, cats are usually animals that love to sleep in comfy places, so it might be a little bit difficult for you to notice a change in this area.
2. Not eating enough or not eating at all. Like all animals, sick cats change their alimentation behavior. Most of them lose their appetite, or throw up after they eat. So, if you see any of these signs it is time to talk to the vet. It might be a simple indigestion, but it is better to treat it from the first signs. Moreover, a change in the eating behavior can, in some cases, be a symptom of a more severe condition than a simple food poisoning.
3. Not drinking water. Not only do sick cats have reduced appetite, but they also refuse to drink water in most cases. Or, there are cases when your cat might drink too much water, being thirsty all the time. This is not good either. Mention this to the vet too.
4. The quality of the stool. This is something not many people want to check, but unfortunately the stool can indicate with precision if something is wrong. Your vet will want to know more about this. The presence of blood or mucus in the stool means that you have to go to the veterinarian immediately.
5. Changes in the way your cat looks like. There are some obvious physical changes that you will definitely notice if your cat is sick. First of all, the nose is dry and hot, if the cat has fever. If the eyes are red and the nose is running, these are also sings to worry.
6. Excessive scratching. If your cat scratches itself excessively, it doesn't always mean that it has flees; it might be a sign of infection or disease. For example, in the case of ear infections, the cat will have the tendency to scratch the area and will not allow you to touch it.
Cats are sensible animals that require lots of love. They do not take too much of your free time and, unlike dogs they do not need to be walked several times a day. However, they can have health problems just like humans, and any of the above signs should draw your attention.
Do not try to treat your cat yourself, without the advice of a veterinarian. You might do more harm than good. Only veterinarians have the right knowledge and experience to deal with sick cats.