You could try antibiotics, or try and see a vet. I think you can get oxymav there, but I’m not sure that it would be a strong antibiotic. If she has a tumor, she may eventually die of it. Can you check with an avian center at a vet college, or speak with a vet locally on how to get a necropsy if...
I would get someone to help, and wrap her in a towel. Sterilize the needle or a disposable scalpel blade, and lance it. Then you would be pretty sure whether or not to do more. If there is a chance that she was pecked or injured herself, it might be an abscess or healing injury.
It may be a skin cancer such as squamous cell carcinoma, but hard to know for sure without a vet diagnosis. Leukosis or Mareks virus can also cause tumors. Usually with tumors, when they are lanced or probed with a needle, it bleeds, compared to an abscess containing yellow solid pus cells.