Hello! i fell off the chair when i was in elementary because my brother plays a prank on me and he pull the chair when i was about to sit. and after i entered junior high school i felt pain on my back and and until now i still feel the pain?

If the pain has persisted so long after that incident I wonder why you never had it checked. I doubt the causal relationship but it can't be ruled out. You should have your back checked, a visit with a spine orthopedist is preferred.

An Orthopedic consult may offer additional information. Physical therapy is also often helpful. Watch for signs of tingling and weakness, but most likely there is muscle strain.