You are right. I can make a lot of intelligent sounding observations but without testing you with movement & position to either abolish or reproduce your pain, the pictures didn't help me to diagnose your problem. It sounds like your back likes extension and movement an doesn't like sitting or stationary positions.
Try to do prone back press ups ( keep hips down and extend elbows) 5-10 times nd see how you move afterwards. Try this when you hurt and not
Mckenziemdt.org check it out