My PT made a joke a few months ago about how anyone coming in to physical therapy for an issue below their shoulders is going to leave with clamshells.

A lot of humans in the modern day have weak glutes and hips and their lower backs will take over as compensation. Try out some glute exercises like hip bridges (make sure to keep a slight pelvic tuck and don’t go up that high to avoid hyperextending your low back) and clamshells. It may be worth it to go see a PT. My guess is it’s probably not an actual issue with your low back and is instead a compensation issue from weak or lazy muscles (the glutes notoriously will choose not to work if they don’t have to)