I agree with 14+kitties. Sounds like an abscess. The poison 14+ is referring to is the pocket of infection. Your kitty may have had a wound on her leg weeks or even months ago that closed up and has slowly festered ever since.
My dog Myka was punctured by a wheelbarrow handle which went in her flank and punched a hole right through her hide. Luckily she didn't damage much inside, but was bruised up and sore pretty bad. The vet closed the puncture and put a drain in her flank and out her belly which drained quite a lot. She still had blood pooling along her belly and down her hind legs. Anyway, long after she seemed all healed up, about 4 months later the original puncture mark broke open with a small abscess that healed on its own with some cleaning. You explain your cat's leg as being quite swollen though which makes me think that you're likely dealing with a much larger abscess if that is indeed the ailment.
Make sure anything you clean the wounds with are cat safe, both topically and internally (of course they will lick it). Cats are sensitive to many medications.
Roxy - Feb 2005 - 75 lb American Staffordshire Bull Terrier (adopted Jul 2011)
Peewee - Jan 2006 - 8 lb Chihuahua (adopted May 2009)
Myka - Nov 1998 to Jan 2010 - 85 lb American Pit Bull Terrier cross
Lacy - Sept 1992 to July 2003 - 18 lb Reg Shetland Sheepdog