Malcolm,
If your lower back hurts in rocks, it is almost absolutely due to a form break down. You should never have your lower back arch and change shape and it should always be in contact with the floor on every rep of the hollow body rock. We will be happy to look at your form if you post your video for review, otherwise, take a video of yourself and pay particular attention to your posterior pelvic tilt, and the shape of your lower back in the rock.