Strange lower back issue started last week.

My back, instead of a flat or slight hump - feels like it is inverting. It causes my center of my lower back to get a bit cramped and after 40 miles or so it gets painful.

I can relieve the issue by going into the drops or standing.

I checked my measurements and they are correct for a fitting (by my chiro/fitter) I had 2 years ago...I know I should go in to have my guy look at it again, but I was hoping for a quick reply or any suggestions (other than go get a fit).

wondering if high bars could cause the issue im experiencing.

(I have been stretching my hamstrings and have gained flexibility over the last few months if that matters)