Stiffness of the cat's body and straight limbs may be symptoms of sudden shock and death. You need to check whether the cat is still breathing. If this situation has just occurred, it is best to send it to a pet hospital for rescue immediately, and it is best to treat the cat halfway. Taking first aid measures can increase the chance of survival. If the cat is sleeping and this happens, then there is no need to worry, it is the cat having a nightmare.