Welcome to the back club. Another topic I know something about. There are multiple reasons you can get back pain. Most importantly is posture. This can be influenced by many things, fatigue, body weight, a worn seat, riding 2 up when you are used to solo, riding solo when you are used to 2 up, sitting on your wallet, changing your seat height, gripping the handlebars too tight and so on. Other things that can effect your back, chewing on one side, chewing gum, dental or TMJ (temporomandibular joint) problems, sleeping on your stomach, standing on one foot, carrying a shoulder bag on one shoulder more than another.
Some solutions: a back rest has helped me greatly, I have the Bak Up unit. A "kidney belt" or some other lumbar support belt can help as this supports the abdominal muscles and forces a straighter posture. Weight loss, exercise and an important concept, stretching exercises. This is good for almost everyone.
If you use highway pegs do they allow for a truly comfortable posture? Does your helmet fit properly? Have you purchased new shoes lately, if so how is your posture when standing in them? Have you ever been checked for a leg length discrepancy (one leg longer than the other)?
Just a few thoughts off the top of my head. Hope it helps.