What Causes Wrist Pain In Children?

My soon-to-be-12-year-old daughter is a gymnast and returned home from a meet yesterday afternoon. About an hour later, she said that her left hand hurt and that it looked weird. When I looked at it, the veins were larger than those in her right hand and were bulging in spots. It looked almost like bubbles in some spots and then thinned down to regular size in other spots. There were about 4 spots like this. After about an hour, the spots still looked different from her right hand, but they were smaller and didn't hurt as much when pressure was applied. Today, I can still see them, but less than yesterday and there is no pain when I apply pressure. She wore wrist guards for the first time yesterday and has been having some wrist pain in her left hand for a couple days as well. I don't know if these are related but I thought I should mention them. Any thoughts about what could be causing this? How worried should we be?