back pain when sitting

Major back pain when sitting up straight?
I play the saxophone, and as you all know, you have to sit with near perfect posture and your back much be strait, toes out, etc.
Well, I didn’t feel pain at the beggining of the year, but now it gets so bad that I will start crying in band over the pain. It also makes it hard to breathe.
I need help, any kind.
Thank you!
It’s obviously because you’re body is giving out. Having to carry that heavy thing all year- any body would hate it after a while. You need to take a break. There could be something worse that could happen due to back pain. Curvature of the spine, anxiety/heart attack.
SITTING WITHOUT BACK PAIN IN YOUR CAR