I hurt my lower back in dance class several years ago. I had to go through physical therapy and the stretch you have in your post was not allowed. I had to do the whole roll in a ball thing that was already mentioned and lots of core strengthening. One of the exercises that helped the most was laying on my back with knees bent, and raising my pelvis up in the air with my feet flat and my shoulders still on the floor. Up and down. It actually provided relief. Hope it gets better soon!