When I Hike Uphill/at Elevations Higher Than Where I Live

When i hike uphill/at elevations higher than where I live at sea level, my chest feels slightly tight; breathing is more difficult - bit of wheezing --- and it feels strenuous. This is a new thing - happens with hiking uphill during the last 2 years. Some coughing maybe also. Clears up once I stop or go downhill. Thank you.