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)

Isn't is usually too much reach that causes this? My lower back pain was similar: happened after 35 or so miles. Had a fitting, shortened the reach per his recommendations, and now is much better.

Did your saddle slip back too far?

Doing core exercises to strengthen your abs/back and stretches to improve your overall flexibility?

Yes. Strengthen your core and abs so that your back doesn't invert. Lower bars might help. They work for me.

Foam roller. Many lower back issues are because of tight IT. GL

When Ive experienced a reach that is too long - I find my neck and shoulders feel strained. Not the case here. I can be in the drops and not feel any strain on my neck and my arms are always bent.

AS far as my saddle - the tip of it is 6mm behind the BB.

I do planks every morning as well as lower back lifts. then a longish stretching routine.

I do notice that if I push against the hoods to force my back to round a bit that is seems to help...

It's sounds like insufficient reach/drop. If your saddle is in the right position, then you should try lowering your bar. If that doesn't yield desirable results, you might consider a longer stem.

I get this problem if my saddle is too far forward. It somehow causes me to sit at an angle I dont want to. Icidentally I have tried adjusting reach and bar height to no affect.

Me too. If moving the saddle further aft doesn't cause any additional pain, then that might be all you need. It's a good idea to try that first. I have found it to have a substantial effect on handling, too.

By inverting you mean like a sway backed horse? I personally try to get into this position by rotating my hips forward to flatten out my back. Pics would help I guess.