Did this issue surface after you were Hospitalized? Can you also also try to describe the " Extremely cold" , do you feel cold as in that part of your body when you touch it or the organ feels cold from the inside?.
Generally to increase Stamina , you will have to do activity that puts a tolerable stress into your lung and body , so it will adapt to handle those stress and you increase intensity as you go. running or swimming is the most common activity to do.