Hi,
Thanks for the query.
During
lactation the caloric need of the mother increases as she should supply nutrition for her self and the baby.
If the intake is not sufficient, maternal stored fat will be utilized to supply the nutrition.
Because of this fully
breastfeeding women will loss their weight very quickly.
If your weight is within normal limits no need to worry.
Usually the ideal weight in kgs should be height in cms - 100.
If you think you are
underweight you can follow measures to increase the body weight.
Take high protein diet chicken, fish, eggs, boiled soybeans etc.
Carbohydrates like whole bread, fruits etc.
For more details : http://srsree.blogspot.in/2013/12/foods-that-will-help-in-increasing-body.html
Better to avoid refined food.
Take care.