Did you follow the full treatment of the antibiotics used, or only used them until it looked like the udder had cleared up? If it's the latter, then it sounds like you did more harm than good. You probably shouldn't have touched that quarter in the first place, seeing as she was already blind in one quarter and usable in all three.
No, she's got a real nasty infection in that quarter. Get the vet out again, and see what can be done other than constant application of antibiotics. And make sure if you do use antibiotics, to go the whole entire length of time required to treat her with, not just until she starts looking better.