Whenever I'm doing a heavy abs workout, I feel it in the lower back before anywhere else.
E.g. I do 3 sets of 15 weighted situps (45 lbs) every week - and by the end my back is screaming for me to stop, while my abs are saying "Is that all? Bring it on!".
This isn't new for me - it's been with me all my life. I remember noticing it at rugby practice, when I was a kid in high school.
Perhaps this is normal and happens to everyone..?