There could be many reaons for the pain, is the pain you have sharp and specific or dull and general? Sharp pain could be bad, The best thing is to start slow, stretch your abs and back before starting you workout. You may also want to try to isolate your abs more by resting your legs on a bench in the gym or against a wall so your legs form a L shape. Then do crunches slow and concentrated. As I said earlier, it is really hard to say, especially since I dont know you and have not seen you do sit ups. Take it easy, and if you belong to a gym, ask a trainer for some suggestions as well, but make sure he knows what he is doing. Good luck, and hope the pain is just temporary.