At 12 weeks old it could have been 'walking dandruff', Cheyletiella, did the vet give you some shampoo, usually Seleen or Malabseb. You can buy Seleen shampoo which is cheaper from the online veterinary websites, but I would be inclined to go back to the vets and ask for tests. I am assuming she hasnt got fleas?
http://www.vetuk.co.uk/index.php?main_page=pro duct_info&cPath=6&products_id=169
Raw liquidised spinach is supposedly very good for itchy skin, but it could be an allergy, and if the vet doesnt know what it is, i would have thought he should have considered allergy tests or skin scrapings