I thought I was having a minor heart attack or something similar. Ruled out panic attacks, etc. Finally went to the doc and it turned out I had an infection in the heart lining - between the muscle and the chambers, basically. It made the tissues swell, so whenever I was under a little physical strain it would put undue pressure on the heart. Nothing terribly dangerous, just hurt a little and scared me more than anything else
A couple trips in for anti-biotic drips and problem gone.