She has olecranon (elbow) bursitis. In this case it is from trauma and bleeding under the surface of the skin. It has persisted because I caught her leaning on a walker with her forearms and pressing on this area.
Olecranon bursitis is common and usually it will resolve. It is important to watch for infection, increased pain and warmth.
Thanks to FP for allowing her elbow to be the teaching case of the day.