Acute sharp pain in bottom left ribs during ab workout

Anybody have any ideas? Started a few months ago. I tried resting for a few weeks and the next time I did abs it came back. I usually do the ab ripper x 2 times a week. It doesn't hurt during any other exercises ( I do greyskull 3x a week). Starts off as a slight pain and quickly intensifies until I have to stop. Feels like a knife going in just below my bottom rib. Pain goes away almost immediately once I stop.