Found strange lump on hind leg

Hey gang, im making this post because im a fairly new ratty owner. Ive taken on an older rat, her name's Toolbox, from a cousin of mine who had become unable to take care of her. Ive done the best i can making sure to feed her well and has all the activities in the world to do (including but not limited to devouring an old robe i left a little too close to her cage once). She's at least three years old.

Anyway, the main point is that today i was checking in on her this morning because she hadnt had as much energy as usual the past few days and seemed to be having some issues eating (her teeth are good and ive gotten baby food for her to eat, no onions :) ) but when i was examining her i noticed this very prominent lump by her hind leg that i definitely wouldve noticed over the past week or so as ive been checking her thoroughly. I dont know a lot about rat health developments especially as they age, so i wanted to see if the community might be able to offer some more information for me. Ive already got steps in motion to have her see a doc, im mainly just wanting to see what possibilities this might be.

I definitely don't rule out a tumor as i hear rats get tumors real easily, but this visibly developed so quickly im feeling doubtful. Im worried this is a digestive or intestinal issue but the location doesnt seem quite right for that.

Thanks in advance for anything you all could offer, ive loved visiting this sub and may get rats in the future, so i hope be able to take part more.