Make sure your lower back is pressed down on the floor. Tilt your pelvis and press your naval into your back, which it literally won't do that. That may one reason, but some of those educated ladies will have some possible reasons why?
Obviously I am not Cathe, but I noticed you were a new user and so am I so I thought I would throw in my two cents.
Perhaps you are not doing anything wrong, but maybe your back muscles are weak. I never would work my abs b/c it hurt my back (mainly lower back)during and after. However, I started using Core Max and also doing the lower back exercises in GS Back, Biceps & Shoulders and my back no longer hurts. I think my back muscles were just extremely weak. Hope that helps!