Check for muscle imbalance. Went through something similar for 3 years. Back doc. , pt. , chiro., mri . , none of them figured it out. Turned out it was her glute medial. The pain was in her sacroiliac joint. Once we figured it out , did exercises for a month and it went away. Now I check all our pitchers periodically. I have them stand on one leg, then the other, checking to see if there hips want to drop on the leg that's in the air. Most kids I have checked are weak on one side. This causes them to mistime there landing(on a micro scale), which causes pain.