I have suffered from some degree of back pain, i found i had loads of tension in my central back after a hour or two of bokken work. I concluded that it was due to me holding unnecessary tension there. I found the following things helped a lot: (although as the professional said, see a professional)
Simply lying on two Jo's, separated by about 2 inches, with thin pillow or book so that your neck is comfortable, not arched back on itself or squashed forwards either.
That is to say - lay with your spine in between the two Jo's.
Breath deeply and relax as much as possible. Trying not to focus on the tension (as in letting your mind look for the pain), but just repeating the works "relax nice back, relax".
I have often found this is gentle way to release tension in my spine and often hear and feel it shifting back into place. A nice feeling.
The other thing I use is two balls, a little bit larger than the size of tennis balls and about the same firmness, Although tennis balls will be fine. place them in a sock or pair of tights. And either: lay on your back and gently roll your spine up and down them, or if this is painful, simple just lay with them at the point it feels the most stiff and then move to a different point. This should/will release tension and allow the spine to gently realign itself.
I AM NOT A DOCTOR - These were told to me by a friend, who was trained as a Olympic gymnast and she pointed me towards a couple of great books:
Conditioning for dance Eric N. Franklin
Alexander Technique By Ailsa Masterson, Aisla Masterson
let me know if you want to hear some more