In Bed, Laying On My Back, I Had A Burning

In bed, laying on my back, I had a burning sensation similar to heartburn, but on the left side of my right lung area, running from about the bottom side of the ribs up about 4 inches. It was similar to having heart burn, but in the chest area. It began, rose upward, and then disappeared. Has not occurred since. This happened after a pretty strenuous treadmill session and some weight lifting at the gym that day.
I am 72. Quit smoking about 15 years ago, but smoked heavily for 45 years or so.