You exercise to feel the burn -- but not that kind of burn. Muscles, yes. Stomach, no. But when you go running, do aerobics, or go to a spinning class, there it is: heartburn. It's not just your legs that are churning, it's your last meal as well, churning right up into your throat. Your exercise heartburn has even made you hesitate to work out and made you wonder: What's going on here?