K.Z.
First off, are they treating your baby for yeast too? If not, they probably should even if your baby's mouth doesn't have any. I had a long run with yeast infections because they never treated my son. As soon as they treated both of us, things got under control.
I agree with some of the other responses that you should get checked out. If it's just a plug duct they can help you with ways to correct it (askdrsears.com has great suggestions), and if it is yeast mastitis it's important to get it treated right away. I would avoid using the cabbage leaves because they are used as a milk reducer and it will permenantely reduce your supply if used incorrectly (I know because it happened to me).
I'm so sorry you are having this trouble. Good luck and I hope you get better soon!