13 year old with extreme lower back pain and when she stretches she fells like part of her spine pops back into place. why is this?

Muscle spasm: advise seeing a physical therapists. They can help you set this straight and provide you with exercises to reduce the chance if a reoccurrence.

See a Physician: Lower back pain is pain that originates from the muscles, nerves, bones, joints or other structures in the spine. The problem may be a simple nuisance but may require urgent medical attention. It is a good idea to see a physician for an evaluation, tests and imaging. See a Physician.