Good Morning and welcome! I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
This may be serious but it is hard to know without your vet veing able to perform diagnostics on this area. It could be an injury or infectious so it is important that you find out so that the appropriate treatment can be given. For now, apply cold compresses to this area every 4 hrs and hose it down so the water massaging thsi area can help reduce the swelling. If you have Butazolidin available, give 2 gms every 12 hrs to help reduce the swelling. If there is no change, call your vet right away.
Does that make sense to you?
Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please REPLY so I can clarify for you.
Sincerely, Dr. Loretta