I think that there's a point, about two weeks after a preemie is born, when the physical stuff can just really start to slam a mama. Your body has some reserves, and that's about the time when they start running out.
Drink water. Eat. Rest. Keep pumping (although I am in favor of a long stretch of sleep overnight). Make sure you always have water with you, and put some snacks in your bag as well.
Tell anyone who asks if they can help to take on your housework, or bring you casseroles.