If I can exercise less and less over the past few months does that mean I have pulmonary fibrosis?

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 likely. Pulmonary fibrosis is a very rare diagnosis. There are many other more common causes of reduced exercise tolerance.
No. Pulmonary fibrosis is a very uncommon condition, but many common reasons for excercise intolerance are, for instance asthma, obesity, being out of shape, heart problems, etc.
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.