Suggest Treatment For Severe Leg Pain

My young daughter age 5 complains of leg pain and says she cannot straighten her legs. I think it is likely "growing pains" and occurs mostly at night, but occasionally throughout the day. Every day she walks, jumps, dances, and does all the things one would expect from a child her age. What should I watch for to determine if there is something that should warrant a trip to the doctor for an examination?