I was about to ask if Erica may have a health condition that caused her short stature, but after some googling, I'm not sure if it could be that she's just shorter (no medical condition causing it or disability).
According to this, the avergae height and weight for a 13 year old girl is:
60 - 63 inches and 95 - 105 lbs.
http://www.disabled-world.com/artman/publish/height-weight-teens.shtml
That makes her about 7" shorter than the average at 13 (according to these numbers, I was 5" taller than this at 13, which is also when I stopped growing), so IMO there's plenty of inches under/over that many 13 year olds are; I don't really know if her height alludes to more going on with her. I hit my final growth spurt early (grew the most between 10-13) and I suspect Erica had yet to hit hers when she was 13.