Does anyone know?

I am worried about my cat. In the last 24 hours he has started acting strangely. I don't know if it is just a phase or if he is really sick. He was growling at my daughter and the dog. (this never happens) And then when I would set him down after picking him up it appeared that he was having a hard time standing, all of his legs were shaking. Does anyone know what this might be? I'm worried, but in the past when I have jumped and just taken them to the vet, it has turned out to be nothing.