As everyone says, please have it checked by a pediatric ortho. It may not be bad, but it's important to not wait. It could be an injury, it could be anatomical, it could be posture. My daughter had intense back pain after she went through a growth spurt in level 5. X-rays and MRI were clean. It turned out to be anatomy (angle of her pelvis) and compensating for it by tightening her gluteus medius. Pediatric ortho referred us to PT and the therapist who herself used to be a competitive gymnast. That was life changing. Our insurance covered everything including the PT after we met the deductible.