If you don't feed pellets, or if the pellets aren't at least 15% protein and alfalfa based, I would suggest adding alfalfa hay to her diet. She'll need the extra protein and calcium while she's nursing. Even though she won't eat dried grass hay, there's a good chance she'll eat the alfalfa hay. Most rabbits like alfalfa hay, though it should only be fed to young growing rabbits and nursing does, and best fed in balance with a grass hay or fresh grass. Like was mentioned, nursing does need lots of food, so just make sure she always has grass to eat, and a good amount of alfalfa hay as well .
If you would like to get her to the point of accepting grass hay, what worked for a stray I took in was drying some of the fresh grass and feeding a mix of the fresh and dried. Then once she was only eating the grass I dried for her, I started mixing it with farm baled grass hay. Eventually I could feed just the farm baled grass hay to her.