Mastitis seems extremely unlikely as a diagnosis here.  You cannot have
mastitis without a hard tender red swollen area of the breast, plus or
minus fever.  Pain alone is still more likely yeast.  An antibiotic is
not only unnecessary, but likely to cause more problems.  The mother
should continue nursing on the affected side.  The physician has no
right to be adamant about something s/he obviously knows nothing about.
And gee whiz, whatever happened to informed choice?

Jack Newman, MD, FRCPC