I would take your dog to the vet and get this thoroughly treated. It sounds very much like a recurring abscess. If so, it will be quite painful for your dog. And if it keeps recurring, AND doesn't get any smaller each time....then that is beginning to get serious. You do not want this infection (if it is that) to spread through your dog's system. Maybe an antibiotic treatment will be recommended if it's an abscess.
Go to the vet. I would go either today or tomorrow.