It seems as if your cat has caught seizure that is epilepsy. You did not mention that if this condition persists or it vanishes after few minutes or hours. If it attacks for only a while then it is epilepsy so you need to consult a vet soon. Otherwise check its limbs and find out if there is any wound there. It can be an insect sting or due to hit by some hard object. It needs a careful examination. In both the cases you should not ignore the problem and take the cat to vet so immediately.