Pepto

I've never had GI issues during training, but during a couple of my races I've had some problems. I get a really bad crampy feeling in my lower back ( like i need to use the restroom) that totally takes me out of my grove. Everything else feels fine, but that. I always eat the same and drink the same as I do in training. Does anyone use pepto or something before the race to help ward off any GI discomforts during the race? I have my first oly this weekend and my only concern is this? Any suggestions would be great! Thanks!