I would go to a vet and get a proper diagnosis, poor thing must be in so much pain and discomfort, get an appointment asap.
it could be anything, is she kept outdoors?
it could be mites, sometimes found in hay.
also the shampoo you are bathing her with might be making things worse rather than better, she might even be having a reaction to that.
unfortunately over the internet no-one can really tell you what is wrong best thing to do is get to a vet as soon as possible