You have not mentioned your height, so it would be a bit difficult to infer if you are underweight or in a healthy weight range.Any way, lactating women at times, tend to lose weight owing to post delivery stress, breastfeeding and irregular sleeping pattern( thanks to the little one!). The focus should be on consuming a healthy and wellbalanced diet than on counting calories.But, if you are losing weight too drastically or feeling weak and tired a bit too often or have gone lower than your ideal body weight, consult a physician.