Harsh back pain helpp?

I play the piano, and it is hard to practice for the right amount of time i would like to. When sitting in the chair, sitting with good posture, my back straight. I have severe back pain. Stabbing in the middle and lower back. it's almost like someone has a VDO doll of me and is poking me there.i can't sit for 5 minutes straight without having my back hurt. I roll on a ball when this happens and i stretch a lot. But nothing helps me. I have been having this problem for around 4 years. I probably should add that when i was around 11 years old, i got double bounced on a tram poling by like 5 adults, and i flew like a bird. i couldn't breath for a while, and something happened to my back. Recently i have ben in a car accident, not to serious, but i'm sure it has something to do with my problem. But i have been having pain from before the carr accident though. I have seen doctors, and done some x-rays, but it is always the same answer, they can't seem to find the problem, or it is just normal. But there is definitely something wrong with my back, it's ruining my life. Does anyone have knowledge of what i have?