One way to know if you're feeling anxious is, are you feeling anxious? Are you worrying over things, are you scared of things in an irrational fashion, are anxious thoughts taking over a larger portion of your thinking. I'm assuming you're running out of the classroom says you're anxious. If the doctor says your lungs are fine, if you're not feeling any congestion, then it might very well be anxiety. It might be time to consult a psychologist to help you figure this out, but a good question to ask yourself is, when this first started, was there something new going on that might have affected you deeply?