Nice job on keeping up with this program! I do hope, though, that the pain you're having is something transitory.
I'm going to see how it feels after the Pig this weekend. I might take a light week next week to see if that helps and then repeat the first three weeks before moving on. My back has not hurt since I got a new chair at work, so I think this seems to be originating from piriformis muscle/hip.
Sounds like a good plan. I find I can usually "fix" these things, but the trick is figuring out where the problem is originating.
Have fun at the Flying Pig!