How long have you been training? Are you well-schooled in the bench, or self-taught? I’m going to make some assumptions below just to throw some ideas at you.

Typically, when bench pressing, the lifter will use a “globally extended” (arched) spine position to create more stability through the shoulder. Doing this properly involves creating a nice, gradual arch through the whole spine, with particular attention to extending the upper back (thoracic spine). What often happens, however, is that an inexperienced lifter (or simply one with less than optimal bench technique) will arch more through the low back, creating a lot of local irritation and, quite frankly, a shittier position than when they started.

There are a few things that might throw your arch out the window, chief among them being your ability to properly set up and maintain global extension during the lift. Most of the mwods with the “bench” tag will touch on creating global extension.

Mobility-wise, if your thoracic spine is stiff, you won’t be able to create much of an arch there at all, which leads to your low back doing most of the moving. Also, if you are missing hip extension (stiff hip flexors), simply putting your feet on the floor (which is just good bench technique) will immediately put your low back into over-extension as your legs pull your pelvis out of place.

I hope something there helps!