It looks quite a bit like what Luie had with how it is growing without disrupting the overlying skin. It could be something like an abscess under the skin, but I'd be more inclined to think it's some kind of growth. There are a few different types of growths. My veiled had some growths that turned out to be papillovirus, and you saw Jann's thread on Luie's growths. The only way to know is to remove it and sent it to the lab for histopathology where they can tell what type of tissue is growing and what's inside. That would of course need to be done under anesthesia since it would be painful, so a trip to the vet to discuss removal would be a great idea. Just make sure it gets sent to the lab even though it's more expensive because otherwise you'll never know what it is. And without knowing what it is you won't know how likely it is to come back or spread to other areas of the body.
