Hi A.! I nursed both my girls and had sore breasts upon weaning for several days. I was surprised too how much more sore I was the 2nd time and I had plenty of milk for a while until my body adjusted. Cabbage leaves do work wonders if that is the problem.
I also wonder though if you don't have a plugged duct. With my second daughter I had them frequently and they are incredibly painful but w/o the mastitis symptoms. If I remember correctly the recommended treatment is cabbage, ice, and massage to open duct. There is plenty of info on line about dealing with them as well. It could turn into mastitis if not opened. Good luck!