It sounds like your dog has a bad rash. This can be caused initially by several things. Fleas are a really common cause. Since you mentioned having your dog dipped, I assume you have seen fleas. You might not have a serious flea problem in your house, but some dogs are very sensitive and have an allergic reaction to even one flea bite. Make sure to use a really good topical flea preventative such as Frontline or Advantage. Another cause of rash in dogs is allergies. This can be to foods, pollen, mold, dust mites...anything that humans can be allergic to. Ask your vet about identifying the cause and medications that can help control this. And lastly, the rash can be caused by the dip itself. That stuff is a chemical that can cause burns and skin rashes, especially if applied wrong.
You should get the dog to the vet for some antibiotics, which will help the healing. A short course of steroids (prednisone) may also be prescribed to reduce the pain and inflammation and allow quicker healing.
I highly recommend giving your dog a bath with a very mild shampoo (baby shampoo is fine) and making sure to rinse all of the soap off really, really well. Then, apply hydrocortisone cream to the spots if they are really itchy. Make sure the dog cannot lick off the cream by putting on an e-collar. If you don't own an e-collar, your local pet supply store should sell them.
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