I would have her checked for mange. If you chanhed the food before this started, it could be a food allergy. If it's mange, you need to find out what type. One type can be spread to humans and from you to other animals.
Vet visit is in order. That could be from a lot of things. Could be mange, environmental allergies, flea allergies among others. The fact you say it's on the dog's back.......if its accompanied by hair loss on the tail and you have noticed weight gain, get her thyroid checked.