Please see your veterinarian. Many times these problems can be cleared up quickly, but if left often result in circular crusts and sometimes scarring without hair regrowth. The skin can only respond in a limited number of ways to a variety of problems, so seeing your veterinarian to have him/her determine the underlying cause and initiate the appropriate treatment is very important. Even if fleas are not seen, your veterinarian will likely suggest using frontline or comfortis brand flea prevention monthly to prevent flare ups that fleas can provide worsening the underlying cause. Please be aware that other mimicking flea products usually don't provide you with the efficacy you need.
I agree with Dr. Mathis, a recheck with your veterinarian is needed. Skin rash/bumps with hair loss is indicative of a skin infection, often bacterial, less commonly microscopic demodex mites or fungal skin infection. Your veterinarian can perform skin scrapings to identify the cause of the infection and prescribe the appropriate medications. Additionally, skin infections are often symptoms of an underlying disease, and in dogs with chronic diarrhea, food allergy is highly suspect, so you may want to talk to your veterinarian about starting a prescription hypoallergenic diet trial (unfortunaely, there are no accurate skin or blood tests for food allergy, the test and treatment are the hypoallergenic diet trial for 6-8 weeks with no other foods or treats).
Kimberly Coyner, DVM DACVD