Not necessarily... There are many possible explanations for less exercise tolerance. If you are experiencing this symptom, see your doctor for an evaluation. Exercise pulmonary testing can be done to diagnose this symptom.
Not necessarily. A decrease in the amount of exercise you can do can be caused by decreasing lung function but could also be caused by a lot of other things. You might have decreasing cardiac function decreasing muscle strength/endurance or worsening anemia just to name a few. Blood test and scans of various types would help to sort out the cause of your decreasing exercise tolerance.