This may sound dumb, but I watch the Amazing Dr. Pol, a good show IMO and he takes care of every kind of animal there is. With that in mind, could it just be a cyst? Seen him drain many of them on that show on all kinds of animals including poultry. One that come to mind is a turkey that had one size wise the same but along the thigh and going under to the belly area. Just a thought for you to ponder. From what you say she is acting normal and does't have any of the symptoms listed in the article