Hello,
At such a young age, it is rare having any of the back pathologies.However as you have given a history of playing the saxophone,so I think that it can be due to abnormal posture while playing it that is leading to all the trouble.Rest of the symptoms that you have mentioned are pointing towards acid reflux disease.
Eat small nutritious meals at frequent intervals instead of a heavy single meal. Exercise frequently and take your dinner atleast 2 hrs prior to bedtime. Also raise the head end of the bed by 2 blocks.Eat more of fibres and drink plenty of liquids.
For now, pls take some antacid like Maalox and Mylanta or OTC medications like zantac or prilosec.Take some anti inflammatory pain killer like diclofenac and apply muscle relaxant on the lower back.Also try improving your posture while playing the saxophone ,so that minimum abnormal posture is there and minimum pressure is there on the spine.If the symptoms persist then pls get an X-ray or MRI lumbar spine done.
Hope it helps.Take care and regards.
Start with a chiropractor to see if it is misalignment of the spine. If the ribs are out of alignment, it can hurt when breathing in and out. If the low back is out, this is an easy fix with a chiropractor. Nerves from your back go to organs in your body. If you are sick to your stomach it could be the middle part of your back out of alignment. If you have a curved spine, you want to see a chiropractor when you are still young so that he can try to fix it. When you wait until in your twenties, they cannot do as much to change the curvature.