Continuing to approach a cat that's in this posture is likely to precipitate an attack. For example, a household cat sitting in the window may see an outdoor cat walk across the front yard. Because he can't attack the outdoor cat, he may instead turn and attack the other family cat that's sitting next to him in the window. Redirected aggression can be either offensive or defensive in nature. What You Can Do If your cat's behavior changes suddenly, your first step should always be to contact your veterinarian for a thorough health examination.
Cats often hide symptoms of illness until they're seriously ill.
Any change in behavior may be an early indication of a medical problem. Spay or neuter any intact pets in your home. The behavior of one intact animal can affect all of your pets. Start the slow introduction process over from the beginning. You may need professional help from an animal-behavior specialist to implement these techniques successfully.