Thanks for replies. It feels like muscle pain right at the bottom of the pack. Started with 70oz, came home with 32.

Still a bit sore, Advil and ice during the night. Have to suspect that over use too (lots of miles I'm not used to over the week-end played into it). More Core work!!!!

Will try a few more rides, with way less fluid to start and also lowering it onto my hips. I have the straps shortened all the way up.

Suspect it could be a bit big for my short body too.