The hormonal changes that occur around a woman's period are often enough to
trigger yeast overgrowth.  Women often get vaginal yeast symptoms for a few
days prior to their period.
A mild vinegar was  of the nipples after feedings (1 tablespoon vinegar in 1
cup of water) might help.  However the severity of her symptoms may require
further investigation into a subclinical (it's there but no symptoms) yeast
overgrowth.
Marie Davis, RN, IBCLC