Liz Buchanan BVSc MRCVS
Hello - it sounds as though this needs to be checked out by a vet. Perhaps there is a dermatitis of some description requiring antibiotic / antiyeast / antfungal shampoo, but if so then these need to be treated with a proprly medicated versions sold by your vet (and perhaps, rarely, pharmacysts after a consultation) not from the supermarket shelf. In the event of superficial skin infection, there are two important questions to answer; what is causing it and how did it manage to grow there - What is the underlying cause? - And how to control any pain? Your vet can start to answer these questions.
Furthermore if there is another underlying cause to this, your vet may need to pick it up quickly. Please give your vets' office a call and ask for this case to be triaged, in order to establish how quickly your dog needs to be seen.