Look back to when it started. What had she just had - new food? new medicine? recent vaccine? new carpet shampooed? If you really can't see anything there, then I suggest it's the food. There are a lot of toxins in commercial pet food and the skin is the main area where dogs (and everyone else) tries to eliminate the toxins.
Go for a raw diet and make sure that you include raw liver and sunflower oil in the diet. Converting to a raw diet can make matters worse for a few days - worse skin, some vomiting, some diarrhoea. HANG IN THERE! This is a clearing reaction and is beneficial. And generally only lasts a few days, depending on the past food and the age of the dog. Despite these symptoms, in practically all cases, your dog is happier in him/herself, showing you you're going in the right direction.
No self promotion please