Breastfeeding and losing weight too rapidly?

Exercise less and eat higher calorie foods. Instead of skim milk drink 2% or whole. Same with yogurt and cottage cheese, get the full fat versions. You really need to be eating more, since you lost all your pregnancy weight, you don't have any maternal fat stores to rely upon, therefore you need to be eating at least 500 extra calories in addition to your normal amount of calories for sustaining your weight and because of your exercise you then need to add more calories for the one you loss while working out. I would also make sure that this sudden rapid weight loss isn't due to some sort of other medical problem.