Havnt given birth yet, so cant speak from personal experience, but have read breastfeeding helps you lose weight quicker. Although you eat a lot, the calories you burn feeding and caring for your baby dont seem to be being replaced.
Glad you feel well, but do think your weight is low. Until your weight is back to usual try higher fat versions of products (eg drink whole milk instead of skimmed), or snack betwwen meals if you dont now. (Even fruit, eg mango, can be quite high in calories)