Was Pixie recently vaccinated? Sometimes the shot goes a bit deeper and into the tissue under the skin, and can cause a lump like that. If so, it's nothing to worry about. But make sure the vet does her other vaccines on her hind leg, as low as possible...rabies/right side, and if she gets the FeLV vaccine, left. Those aren't needed unless you are at high risk for others coming in with FeLV, and exposing Pixie to the virus. She's an only kitty if I'm not mistaken? If not, maybe rough play caused an abcess?