Hey man, unlike other areas of the body, the back is...touchy, for lack of a better word. Back problems seem to run in the family...although I am surprised because you've lost weight not gained it so if anything, your back should be thanking you. Its picked a strange way to show it though.
Do any heavy lifting beyond fighting and re-enactments in armor? That alone could probably have a lot to do with it. Sucks growing old, doesn't it?
I wake up daily with aches and pains that I swear are brand new.