What Does Sharp Pain On Side Of Body Indicate?

Father with a daughter, age 15, approx 5-5in, 110lbs. No allergies, hypothyroidism, levothyroxine daily. Runner - experiences a med-moderate (lev 2-4+) sharp pain in her left side - within 100-400 yards of starting jogging. 98+% occurrence, consistent regardless of light effort or moderate effort. Ran Cross Country - so "in shape, conditioned", NOT seemingly "side stitch" issue. Taken in for x-ray/sonogram, blood workup, nothing abnormal detected. Father is at a loss on ideas to pursue. She is so wanting to run without pain. I know hard to offer suggestion online, is there particular type of doctor/physician I should focus on - or actual tests while running on a treadmill to monitor changes in body.