Short answer: High altitude + Hiatus hernia both are responsible. Elaboration: altitude sickness though generally occurs above 8k feet , it may occur at ht you mentioned. Specially when you ascend at speed of 500 ft / day(or more) and oxygen concentration is less. In my opinion, hiatus hernia is more responsible for your breathlessness, as herniated stomach bulges(more after you eat/drink) into the lung and decreases inspiratory air volume....and thus reduces resultant availability of oxygen. If your symptoms are not relieved... Consider Decent + visit nearby ER. Take care.