I would almost bet that it is an abscess from all the scratching she has been doing! Or maybe another kitty scratched her. They fill with pus very quickly! It will most likely be lanced and drained and she will be put on antibiotics. She is most likely running a fever while her body is fighting the infection. It will be rather nasty and oozing once it is opened, but it will feel much better and heal rather quickly. You will probably get some wound care antiseptic to put on it as well!

edit: Oh yes, you probably need to find out why she is scratching so much. If not fleas, it is most likely earmites!

Hugs to you and kitty!
Kim