your dog had an allergy to something in its environment. Puppy licks paw. Constant licking makes some area of the paw raw. Puppy chews paw due the burning sensation. Paw gets infected.

Take puppy to the Vet. They will probably prescribe an antibiotic, plus an antibiotic salve. Also ask the Vet about treating with an antihistamine to counter any allergies. Usually Benedryl (1-2 mg/lb) does well.