My daughter is complaining of a sharp pain on the right side of her stomach after 10 minutes or so of easy running. It is slightly below her rib cage. She is doing 28 or so miles per week, and the problem just recently started. It's a sharp pain that causes her to have to stop running. Any ideas on a cause? I'm thinking a muscle strain or perhaps gas buildup, would appreciate if someone could shed some light based on their experiences. FYI she's 13.