My 19 yr old cat has started to bite me in anger while I sleep. He has always slept with me, this is something that he just started doing and I don't know what to do about it.
There are a few possible reasons of it:
Disease. Your cat is rather old, so he might have some health problems, suffer from pain. Unfortunately, animals can't say what bothers them. The way they communicate with us is by their actions and changes in their behaviour. Some animals start peeing everywhere, some don't want to eat food they usually eat, some of them become aggressive. You should take your cat to a vet.
Again, reason, that is connected with your cat's age - changes in his mind. Amentia I would say or dementia. Only after visiting a vet you will find it out.
New members of your family. Maybe a boyfriend/girlfiend/baby, etc. Some old can't start feeling loneliness or they feel how your attitude to them has changed. Or it maybe a passage or stress.
In any way, I wish your cat is fine.