What is wrong with my cat?

Probably mange. My cat had this problem once, so I took her to a vet. she ended up having an allergic reaction to the food along with it. since you already changed her food, then I'm almost positve it would be mange. mange doesnt just go away like a cold or something. it can be fatal. I would take you kitty to vet immediatly. However, it could be getting into fights repeatedly with another cat, and this other cat could be giving wounds that are infected, or ping on its disease to yours. Go to the vet and get it checked out. hope this helps! Good Luck! :)