I have a dog who's 7 and a complete stupid idiot. I have a cat who's 1 and a bit evil, the cat and the dog live happily together if the dog stays away from the cat. Why has the cat suddenly started hissing at the dog when he comes back from his morning walk? The poor sod is terrified he's currently stuck in my bedroom as she's standing in the door way. Any advice how I can stop this!?