I am not a vet, but I am a vet tech and I'm just taking an educated guess by your description. As others have mentioned it sounds like a cat fight abscess. It needs to be opened, maybe try a warm compress until you can pull the scab off and drain out all the pus. This needs to be kept open if it is indeed an abscess until the pocket the pus is filling gets a chance to heal. You can use hibitane soap (available at any drug store) to clean the area twice daily, you can dilute it with water and use it as the compress as well. Good luck, and seeing a vet is generally the best idea.
:eb: JJ and the Zoo