Trouble breathing, feels like I'm not getting enough oxygen?

This could very well be asthma even if your breathing in the Dr.'s office was fine. Most Dr.'s won't take anyone seriously about breathing issues unless they see it for themselves. Mild asthma can be intermittent just as you describe and can result from allergies or cold air and can be made worst by certain medications like aspirin, ibuprofen (Advil), naproxen (Aleve). The next time you begin to feel it coming on try breathing through pursed lips, so, that you have to force the air out of your lungs. This naturally tends to open-up the bronchi and may help at the time. You should be checked at the time that you are having symptoms or go see an asthma specialist that has the equipment to really test lung function.