If you're nursing, your body requires slightly more calories than it did even in your last trimester of pregnancy (about 500 calories more per day versus 450 per day in the third trimester).
But even though you need some additional calories, the additional hunger is often intense. Pair that with lack of sleep and you've got a recipe for overeating, which can obviously lead to slower weight loss and even weight gain.
To prevent overeating, focus on eating foods rich in protein and fiber to help keep you satisfied. An easy way to ensure the protein-fiber combo is to stick to the plate method: Make half your plate veggies, one-quarter whole grains (or starchy veggies like potato, sweet potato, peas, corn, winter squash, etc.), and one-quarter protein. Add about 1 tablespoon of healthy fat either via cooking oils, salad dressing, nuts, seeds, or avocado.
Factor in one to two snacks a day, and, of course, leave room for a small treat!