What could my back pain be?

When I was about 14 (I am 20 now) I experienced severe back pain in a very localized spot. On the right side, to the right of my spine (almost considered to be on my side) about 2-3 inches below my shoulder blade. The pain isn't sharp or throbbing, but it's tightness and pulling and it's extremely uncomfortable. It's okay when I am laying on my stomach or back or sitting with something against it, but sitting without anything or leaning over is terrible. I was very active back then and when I was doing physical activity it felt fine. Since then, it has come and gone. I had it x-rayed and it showed nothing. I did not have an MRI as the doctor wasn't concerned. I do not have scoliosis. It is back again and I just don't know why! It feels better with heat, ice, and rubbing it. There is no mass there visibly and I can't feel anything. What could it be?