I have two cats, a boy and a girl who are now just over one, we got them 8 weeks apart. They've always got on okay. I've just had the boy neutered and have been letting them both out. Since then the female has become very aggressive to the male whenever he comes back in, hissing and trying to attack him. She hasn't been neutered yet as she has shown no signs of being in season. Has anyone had this before?