Lower Back Strain

Every time I surf, my lower back muscles start to strain because of the arching of the back while paddling out. I already tried repositioning myself on the board but that just ruins my paddle efficiency and even then (after a while) it still aches. It usually happens when I have a long paddle out. I've tried strengthening my lower back for the past month now but it still aches -- which I don't understand because my cousin never complains about his back and he never works out his lower back. I know I'm doing something wrong, i just don't know what. How can I change my paddle so my lower back stain doesn't ruin my surf session?