My male chihuahua has had episodes where he will be lying down and I call his to get up and his hind legs do not move. At first I thought it was a seizure but he is fully awake from the waist up when it happens. It has happend about 7 times in 5 years. I asked my vet but he didn’t think much of it. Now my older chihuahua just had the same thing happen to her. She had just gotten up when my husband called her for a walk. She walked down the hall, sighed and dropped sideways. Her upper body was fully functional but her hind legs were "stuck". This is the first time that it happened in 5 years. What is going on?