What Causes Recurring Shortness Of Breath And Palpitation?

Hello,I have a chronical shortness of breath every night around midnight(although it happens in another time of day or night) The symptoms are rapid shallow breathing with accelerated heart rate I am gasping the air and shaking. This episode lasts several hours till it calms down/ All examination so far(heart and lungs) showed no abnormality and relation with shortness of breath.