The first answer that might spring to mind is fleas, but if your veterinarian has recently treated your dog for the pests then this may not be the cause for scratching. Your pet could be suffering from a skin infection such as yeast dermatitis, which VCA reports is a very common cause of irritated skin among canines.
Yeast dermatitis is a fungal infection that takes advantage of a change in skin conditions or a suppression of the immune system. VCA reports that a dog's skin is a natural host to bacteria and fungi, but if the balance is upset, yeast can take over and cause infections. Clinical signs of yeast infections include dry, flaky or scaly skin. Infected skin may also become thickened, turn red or develop a dark pigmentation. Some dogs may also experience chronic ear infections or their skin could develop a musty, yeasty smell.
Yeast dermatitis is not infectious between pets. It may result from a pet receiving dog shots of corticosteroids or a weakened immune system. There are a number of breeds who are genetically predisposed to yeast dermatitis—including poodles, many different breeds of terriers, cocker spaniels, Basset hounds and Dachshunds. Yeast dermatitis may occur in dogs with hormonal imbalances—like hypothyroidism—or in those with food or airborne allergies. The presence of yeast skin infections is usually a sign of a more significant underlying medical issue.
If you suspect your dog has a skin infection, you should bring it to the veterinarian to determine the underlying problem. A pet health professional can prescribe either a topical or oral treatment which can help relieve your dog's symptoms.