If your doing crunches with poor form that may cause some lower back stress. I myself don`t do crunches because I feel my upper abs get enough work indirectly from other exercises such as deadlifts, squats etc. I concentrate on different types of leg raises such as knee-ins on a bench, hanging leg raises, bent leg raises etc. which are easy on the lower back. I usually just work my abs once a week for 20-30 minutes.