Not all women lose weight with breastfeeding. I was one of them! Just pulled an article of the internet. Hope it helps!!!
Average milk production is about 750 ml (25 oz) per day and the energy required to produce that milk is about 200-500 calories per day. Your body has been storing fat (especially in the tummy, hip and thigh areas) for 9 months in preparation for breastfeeding.
Nursing or pumping moms will often retain an extra 5 or 10 pounds until several weeks after weaning. This additional body weight protects mom's ability to produce milk in cases of illness or a severe reduction in caloric intake. However, the mom who is gaining weight while breastfeeding should closely examine her diet for "empty calories" and increase her activity level.
Weight reduction diets shouldn't be started before 6 weeks postpartum. Liquid diets or other very low-calorie diets are not recommended because they're basically starvation diets. When mom is deprived of calories, her body fat will be retained and lean muscle tissue will be used for her milk production instead. When mom returns to her previous diet, her body may then store more fat as a defense mechanism against future episodes of starvation. Take home: Eat a well-balanced diet, start with some light or moderate exercise and pace yourself -- fad diets or starving yourself will not be helpful in the long-term and they may in turn, impact your milk production.