Breastfeeding and fasting combination is a "no-no!". Your body can't give your baby electrolytes and food and in the same time keep them for itself. Also, I suppose, breast milk would go somewhat toxic due to the cleaning process.
Fasting is for you, but not in a breastfeeding mode. Isn't it written in the Bible? Breastfeeding mothers and pregnant women should ABSTAIN from fasting?
DL