Anxiety based shortness of breath does not require activity to trigger. So, you should check into COPD (did you ever smoke?).
When this happens to me, I get freaked out even though I know intellectually that it is not real.
• Drink some water
• Use a humidifier and stick your face in it and moisturize your lungs
• Take in a giant lungful of air thru your nose and exhale slowly through your mouth until your lungs are completely empty. Do this a few times.
• Try some anti-anxiety medication, prescription or not. It may help you psychologically if not in reality, so that is a solution.
• Distract yourself by posting here and use a hotline if neccesary.