I second what Mike wrote - it is quite possibly allergy related either from food or shampoo you are using. My oldest yorkie has skin allergies and dumb me kept feeding her food with corn in it!!! She is on prescription food now that has absolutely no corn or wheat in it and the allergies have cleared up
Check with your vet on possible testing, inquire about foods without corn or wheat in it, and you may need to find a hypo-allergenic shampoo. You'll pay more for the top quality food but it is so worth it because your precious baby will be so much happier
Best of luck to you!