It's hard to be sure over the internet, but it sounds like you may be describing a bite abscess. Does your cat go outside? Another cat may have bitten her on the head, a deep bite that basically injected bacteria into the wound. Then the bacteria got trapped inside. So it started to heal on the outside, but then all that bacteria just makes it burst open. The area can be disturbingly large.
You should take her to the vet, just to find out for sure what it is. If it's an abcess they'll have to clean out any dead tissue and clean it out, then put her on antibiotics. You need to keep her inside so flies don't land on it and lay eggs in the wound. That, my friend, is a biology lesson you don't want. Trust me. : )
I wish you the best of luck. I hope your kitty is feeling better in no time!
