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Your dog may have allergies to food or airborne particles.(rag weed, pollen,grass,etc) Generally dogs will lick and scratch with allergies. If the licking is excessive the skin pigment can change. Dogs with allergies may develop Atopy which is basically another skin problem. I am going to direct you to sites on this for further review.
Also the German Shepherd part of this breed is prone to pyoderma which is a bacterial skin infection. This will show symptoms of itchy skin lesions, pustules and draining sores. If you see this happening then that is probably what is going on.