You get short of breath and feel like you can't breathe. Severe anxiety ! You need to see your Doctor and get a prescription for an emergency inhaler. I have one with me all the time ! It can be treated with an inhaler!
Try placing your nose and mouth into a paper bag and breath normally until you can no longer breath. It's sort of like that, except you would also feel pressure in your chest. You have to calm down,take deep breaths, avoid high humidity and strenuous activities. Most asthmatics need an inhaler to stop the attack.