you don't say how much you are feeding her. I have an anery corn called Marmite who is around the same age (hatched 6th Aug) He is currently around 22-23 inches long and has 3 assorted size pinkies once a week.
To be honest, if she is looking good and healthy and not constantly looking for food, the fact that males are supposed to be a tad bigger than female, I wouldn't stress too much
Warrensark Stud (rats) Southsea
1.0.0. Cockatiel: Nelson Dilbert, 1.0.0 Blue tongue skink: Worcester, 0.0.2. GALS, ,10.20.0. Rats
1.2.0 Corn snakes: Marmite, Bovrilina & Oxo
0.1.0 Hog Island Boa: Bisto, 0.0.1 Bosc monitor: Colman and 1.0.0 husband