I got mastitis in one or two quarters of one breast the other day (due to plugged ducts). The fever went in less than 24 hours so I didn't go to the doctor (plus I doubt they know much about breastfeeding!).
I got my daughter to feed off that side a lot, had a hot shower and massaged the breast, and used a manual pump on it. Because the fever and redness had gone and the lump was getting less hard, I assumed she had unblocked the duct(s) and was clearing the milk.
But she has emptied the rest of the breast very thoroughly today, and I can feel a big hard lump (with smaller lumps on/in it) still on that side.
Perhaps the ducts didn't clear, and the cells are undergoing involution instead? If so, will those quarters relactate eventually as she'll still be feeding off that side? Or will the other quarters just increase their supply to compensate?
I will ask a LC if nobody here can help, so don't worry about suggesting that.