My little working cocker Lizzie, yelps when I am brushing her, I am very gentle and and can find nothing in the place where I was brushing. Sometimes I have barely moved the brush over her fur. She is especially sensitive around her rear end, but yesterday it was around her chest. I can spend hours brushing and combing her ears, she loves that. Does anyone have any idea what is the matter with her, and what I can do about it.