Practice annoyance: unexpected acid reflux - cause/remedy?

Has anyone else experienced this? I have never in my life (52 yrs.) had any problem with heartburn or acid reflux. But often when I'm doing my swim practices, and pushing hard, I get something like acid reflux - a slight burning in my esophagus. At times it persists throughout the day. I never have a problem on days I don't practice. I've thought of experimenting with calcium pills before workouts, since I know working muscles hard uses a lot of calcium, but I have no basis for thinking that is related. I'd be interested in any thoughts on its cause, remedies, etc. It's a minor annoyance, but striking for me since I've never had problems of this nature ever before. Thoughts? Experiences? Solutions?