Hi BWDave
as a physio I would suggest that you stretch your Psoas, this is the main hip flexor that also attaches to the spine. You do this by kneeling

add to this image by stretching (as if to ask a question) the same arm that is on the same side as the leg you are kneeling on and lean away from that leg. ie - if you are on your right knee, stretch your right arm up and side bend over to your left - this will add to the stretch of this muscle.
the 2nd stretch would be :

this is an effective lower back rotation stretch.
Keep in mind the lower back or lumbar spine is not designed to rotate - your upper back or thoracic spine is. The lower back is really just involved in side flexion. If you are excessively roated at your lower back with 'short sharp intense' strokes - this is a sure fire way to get an injury, so I would echo Jodie's advice - review your technique.
Hope this helps, let me know how you go,
Cheers
Gooch