Pregnant cats lose weight after giving birth. However, their nutritional needs increase dramatically. Energy needs can be two to three times normal, depending on litter size, in order to produce the milk supply that will support the offspring. Water intake is also important for milk volume.
To ensure a nursing cat is getting enough nutrition, give her a nutrient-dense diet, such as kitten food. Without increasing the amount of food at each meal, increase the number of meals in the day. Free-choice feed her, offering unlimited access to dry food.