My girl did that when she had her baby. When she leaves him for a little bit she will periodically call out and until he meows back. It's a mama way of communicating, however I would check her out further. First things nurses do is assess the situation. Does any of her breasts look swollen and feel hot to touch? Is there anydischarge from her vaginal area? If the kittens healthy and eating properly (appear to be getting bigger)? Any one who has a new mom cat and newborn kittens needs to watch their cat closely. Cats could get an embolus from a piece of the placenta that travels to the lungs and kills them or they could get an infection in the breasts or uterus which is fairly common. Keep an eye on her if she continues to act oddly I would bring her in to the vet or at least call one to see what they want you to do.