Those look real good George, looks like you're getting the hang of knapping!
Hey, I had some back issues last year and thought I might share what helped me. Mine wasn't caused by any specific trauma that I know of. I was just crouched over patching some concrete in my basement floor one night, and the next morning I woke up with an aching back. It persisted for about 6 months. I went for about 30 chiro "adjustments" and that didn't seem to be doing anything. Just when I'd get a few days of reduced soreness, I'd do something to aggravate it again. I finally got enough sense to go see a physiotherapist and she gave me some basic stretching exercises to do. Didn't really seem to help at first, but once I started doing them on a regular basis 3 times a day, all my aches disappeared. I was quite surprised. Might be something to look into if you haven't already.
Take it easy anyway buddy. Hopefully it improves soon!