Why would you under-nourish yourself while you are BREASTFEEDING!?!?!?
Your personal weight should be the least of your worries when the sole nutrition your baby is getting is coming from YOU.
Goodness. Eat more!
I stopped losing weight and my milk started to dry up when I was eating 1,500 calories a day while nursing. I wound up having to eat roughly 2,000 calories a day to lose weight and continue nursing and I didn't even exercise.
Gah. Since I can't edit my post for some reason...
Use this calculator to see how much you burn while breastfeeding. It's the most accurate I've used that takes nursing into account:
Also, nursing women are usually told to eat a minimum of 1,800 calories a day and most need more.