I've lost 40 lbs while nursing. Not including the 23 I lost in the first 6 weeks post partum - hey maybe I should include that 63 lbs sounds impressive...
Anyway, yes, you need to eat more than you would eat not nursing - there are a couple different sources that actually calculate it by weight and age (the baby's not yours
) but 500-800 calories is a decent approximation.
I lost most of it rather quickly eating about 2300 calories a day but I exercise a lot.