This could be an abscess or a tumor. The only way to determine the difference between the two would be to have your rabbit examined by a veterinarian. Remember, this website is not designed to replace your relationship with a veterinarian. There is no substitute for seeing and feeling the pet to determine the diagnosis, and treatment plans can be too complex to cover in their entirety in this venue. I am happy to share our experiences here at the Sanctuary and offer second opinions based on response to therapy, but please seek a live opinion first.