I wouldn't worry too much right now. It's winter, and most snakes slow down and decide not to grow. My snakes for example usually shed every month, but in the winter, they shed once every 3-4 months. They also eat less, 2 small fuzzies compared to 2 hoppers for my yearling.
If she's active, skin looks good, and is generally healthy, I'd just wait till summer and let her grow then (some snakes grow like weeds over the summer months. My yearly grew 6 inches in something like 5 months).
If your snake is lethargic, has strange poop, looks dehydrated, then I'd take her to a vet and get her checked out. Parasites are most common cause of a snake not growing.
hope that helps.
-13mur 6