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Based on your description, your kitty is most likely suffering from an abscess. An abscess forms when an animal is scratched or bitten, which introduces bacteria underneath the skin. The wound on the skin then heals, leaving the bacteria trapped underneath. The bacteria then begins to multiply, and as it does so, the body sends certain types of white cells to the area to try and fight off the bacteria. As the white cells and bacteria die, pus is formed beneath the skin causing creating the fluid-filled lump that you are feeling.
I would recommend taking your kitty to see her Veterinarian, especially since she is an older cat and her immune system may not work as well as it once did. Without treatment she could develp a very high fever and become very sick. Her vet will be able to open the abscess to drain and clean it, and will probably prescribe her some antibiotics to help her recover. Treated properly, most cats recover well.
Thanks for your question, and please let me know if I can be of any further help.
Best of luck!