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Many times a rash may be present if the razor is hot or the dog shaved too close.
If the skin is itchy you can try a dog cortisone ointment. Sometimes dogs that are often groomed and trimmed they develop coat funk where the hair does not grow back for a couple of years in some areas, but if the hair is falling out and the rash growing you will want your vet to do a skin scraping to determine if it is some type of bacteria or fungal growth possibly picked up by the groomers equipment. Ring worm would come to mind as a fungus that would show hair loss.
Thank you. IF it is circular you want your vet to check it for ring worm and this is contagious so do not touch the area with your bare hands as it can transfer from dog to human. Here is more information on this http://www.petplace.com/dogs/ringworm-dermatophytosis-in-dogs/page1.aspx#
Another possibility that happens with shave downs is coat funk. You can see pictures here