so all ab exercises give you back pain or just some? I have never really had back pain doin crunches and things of that nature, sometimes exercises like leg raises and things can. Easy fix for that is to slightly lift you head while doing the exercises.
Another thought might be to try doing ab exercises on a swiss ball. Or try strenthen your core doing static exercises like plank and holding your legs out in front a few inches off the floor. These exercises will give you a good base from which you can move onto other ones.
Hope this helps
