i doubt it's serious at your age. I'm 18 myself. and the obvious reason your coughing up blood is because you smoke way to much and your throat and lungs can't handle all that toxic. when u cough a lot you break little blood vessles in your throat that leak blood causing some blood to be in flem. But overall the bottom line is you really need to cut back as much as possible unless your purposely trying to KILL YOURSELF.