Bald patch on my puppy's rear?

She is 9 months old and has a docked tail. The patch starts from the end of her nub and stretches toward her back about six inches in a 1-inch wide straight line. I've just inspected it, so I have not been to the vet yet. There is no bleeding, fleas, or roughness. The long hairs are all that is missing, and the soft, lower layer of fur is still there. She does not seem to be scratching or chewing that area with any kind of emphasis. She has a healthy diet and weight. She is mostly an indoor dog spending less than 3 hours outdoors per day. I have not looked into it until now because it looked like just a weird cowlick. Anyone know what's going on?