Oooh I might make cookies today too! People are moving into the house across the street, so maybe I will bring them some.

I wonder if maybe you're subtly adjusting your posture because of something else about the pack. As in, the pack isn't directly causing the pain in your low back, but you maybe hunched one shoulder slightly because of the strap and inadvertently caused muscle pain in your back instead. I'd suggest when you take it out again, loosen all of the straps completely before you put it on and then try to tighten them once you're in riding position, if possible (rather than putting it on and adjusting while standing up, and then getting onto the bike). There's definitely an art to getting the straps adjusted correctly.

(I'm no medical professional, but your description of the pain sounds like mine when I'm having problems with some of the deeper stabilizer muscles, which shouldn't be aggravated by the pack itself but by your posture)