I think there is some change in your home that is making your cat upset. Well if you are sure that there is nothing like that then still pay more attention to your cat and give it more time daily. There is a disease known as Toxoplasmosis. In this disease cats show brain disorders like lack of coordination, circling, constricting pupils, personality change and other central nervous system abnormalities and jaw quivering etc. You cannot do anything if your cat is suffering from this disease. All you have to do is take it to vet and give it medicine regularly as prescribed by its vet.

 Cats quiver their jaw if they are hunting something tiny like a fly
if that's not so then it might have something stuck in its teeth or sometimes flat pieces of bones can lodge on the surface of the upper gums and that might be irritating your cat