Found out it's a small tumor and had it removed that day. They had it sent away to be tested for cancer, but her blood work was OK so it's probably either not cancerous, or if it is, it hasn't spread yet.
Her incision is about 2 inches long and she has a big shaved rectangle on her leg, so of course she's adopted various names like "FrankenPup" and "Naked Butt Pup." Her stitched come out in 10 days.
Out of curiosity, how would you treat a dog for cancer?