breathing problems

60 years old (just). Regular exerciser (running, nordic track), trim, no known health problems other than slight mitral valve prolapse diagnosed 20 years ago but echos now clear. Live in the south where 100 plus degrees for last 10 days or so. Heart cath done 3 years ago was clear. Smoked a pipe (inhaled) until age 42. No smoking in last 18 years. Live in medium size city (.5M).

Problem: In last several weeks become slighlty winded when climbing stairs. Still exercising but becoming winded just after a flight or two of stairs is a new development. No chest discomfort or other symptons. Just shortness of breath after fairly slight exertion. Any thoughts?

Thanks for your reply. What causes stomach acid into the lungs? How is it treated? Is it a one time thing or repetitive? Thanks