What Causes Breathing Difficulty?

I am in the army. I have been having difficulty breathing for about the past month. It feels like I am not gettin enough air and I get winded from the smallest tasks such as walking up stairs. I had a Pulmonary Functions test done and it came back normal. I was told by someone that it may be asthma, but the military doctor I saw said there was no way it was asthma cause I do not have family history of asthma and my Pulmonary Functions Test was normal. Could it still possibly be asthma, or what do you think it may be?