Welcome to HCM.
It is very difficult to precisely confirm a diagnosis without examination and the answer is based on the medical information provided.
based on the information provided by you it appears to be an abscess
. As it is of the size of lemon and not responding to antibiotic th best option would be incision and drainage
. I advice you to see surgeon for examination and also for I and D, after this they will give you antibiotic and pain killers to reduce the pain.
Hope I have provided the information you needed.
Wish you good health. Take care.