You can try support feeding her Critical Care, a powder that you mix with water or baby fruit purée or mashed banana. It has all nutrients a bun needs + a probiotic and it’s recommended by vets for buns that have difficulty keeping weight on.
Since she’s so young, you can also try alfalfa hay. It’s rich and very palatable for rabbits.
How does her poop look? Here’s a guide to bunny poop with pictures: http://imgur.com/a/5N4lD
Bunnies can have intestinal parasites (worms). It’s not super-common with indoors bunnies, but it does happen since they do eat stuff from the outdoors. There are several dewormers suitable for rabbits. Some are broad-spectrum and some are more narrow. To determine what type of parasite might be the culprit so you can chioose the appropriate medicin, your vet can do a fecal exam. Here’s a list of bunny-safe anti-parasitics: http://www.medirabbit.com/Safe_medication/Anti_parasitics/safe_antiworms.htm
(The brand names are often not the same as the substance name listed in the table.)