Mine turned into "demon dog" right after dinner, biting, running around and teasing my Aussie. I read that puppies when they get overtired will do this. She wasn't getting enough sleep because I would put her in a playpen while I worked at home, and every time I left the room or made a noise, she'd wake up and cry..so her sleep was interrupted all the time. Now I put her in her crate in another room at the same time she would have gone to sleep in my office room, and then she sleeps twice a day for a few hours. Made all the difference..