I get hungry when I work out heavy, and sprinting is definitely part of that. Give her more food. Fat calories don't quite always do it for me when I work out hard, so sometimes I need large quantities of potato or rice.
If you're concerned about it, have her drink some water and wait 20 minutes, but my bet is that she needs more food. Even if she isn't growing taller, she's still 1. developing and 2. growing muscles. That'll take a lot out of her.