What could be causing me to have a hard time breathing all of a sudden?

Read the list: Pulmonary embolism, myocardial infarction, pneumothorax, ruptured chordae tendinae or acute pulmonary edema from any other cause. Less serious causes: panic attack, hyperventilation syndrome, asthma. Other rare causes: cyanide poisoning, cardiac tamponade, hemorrhagic shock, acidosis from any cause. I've probably missed a few causes but that's a starter.