I have had my cockatiel for 3 weeks. l purchased him at a pet store and realized I would have to work hard at taming him. He was 8 months old when I got him. I thought I was making progress with him. All of a sudden today he is nippy and hissing. He had been whistling and appeared to be happy. Today, along with the hissing and nippy behavior he has been very quiet. Any Ideas as to what might be going on with him?