So is it gut area or guy area? Big difference. I never have a poop feeling in my guy area so I'm guessing you mean the former? One thing's for sure, it's not lactic acid. For one thing, there's no lactic acid in our bodies for more than an instant, because it immediately changes to lactate. Another thing is that while everyone yaks about lactic acid or lactate, we have no issues with it. Lactate is fuel and is good for us. The chemical that causes that burning feeling is something completely different, can't remember the name though. Nice thing about lactate is that it's easy to measure and increases at the same rate the the chemical which causes the burn. Which is neither here nor there anyway, just FYI.

What's probably happening is that your getting too much stomach acid, like in the old TV commercials, "acid stomach." So take an antacid. They buffer that acid, much more efficient than dosing yourself with a base. Gaviscon works really well. Probably has something to do with what you're eating before and during your races. Try something that's mostly sugars or maltodextrin. Eating too much or eating the wrong thing, combined with going really hard will often mess up one's stomach. Another thing that works well is electrolytes and more water, dilution.