Did your Vet aspirated the lump? If not, you should ask him to and send it out for pathology. Hope it is just infection but could be Lymphoma
Is the lump hard or more like a pocket of fluid? Can it be drained? This is for the vet to decide, but it seems he wants to try standard antibiotic.
Our Westie Terrier developed a sudden hematoma on one ear. We took him to the vet and it was drained plus was put on antibiotics. The dog may have a permanent bend in the erect ear, but the pocket of fluid has not returned - been at least two months. Our dog has also developed lumps on his face and neck, just two so far. They were removed with surgery and biopsy done. No cancer and these have not returned.
I have no way of knowing if this relates, but it does suggest there are more aggressive measures that may be indicated.