You need strength and stamina to meet the physical demands of caring for a new baby. Although your baby won't be harmed by occasional dietary lapses on your part, if you don't get the nutrients you need from your diet your body will draw on its reserves, which can eventually become depleted.
It's natural to feel extra hungry while breastfeeding, as your body is working around the clock to make breast milk for your baby. Eating small meals with healthy snacks in between (the way you may have done during pregnancy) is a good way to keep your hunger in check and your energy levels high.
There's no one-size-fits-all answer to how many calories you need to consume when nursing. As a general guideline, most women who are breastfeeding will need about 200 to 500 calories more, a minimum of 2,000 to 2,700 calories per day.