Fell on butt on concrete really hard nov.12 my back hurts really bad what could be the cause?

Back pain in young: Young patients rarely need further evaluation for back pain as it is usually self-limiting. But yours has been persistent. Consider disc herniation which may cause pain without sciatica in young patients. Stress fracture occurs in 5% of the population without any symptoms, but may be aggravated by such a fall. Since your symptoms persist, see your doc for a more specific evaluation.

Injury!: Could be a disc problem, fracture less likely, or soft tissue injury. Get an x-ray and get examined.