Blood in phlem

I am a 24 year old male, pretty healthy, I eat well and have a normal appetite. Due to pot smoking I am aware that I have lung damage. Last year I went for a lung scan and than went into the tube as well were the doctor said I had lung damage, foolishly I kept smoking, but at a much reduced rate. About a month ago I had a lung scan which showed several white patches on my lungs, the lung doctor looked at this for some time but pronounced everything okay. I have now stopped smoking altogether for about a month and recently have coughed up blood with phelm. I don't cough much, I do feel lung pain sometimes, but I sleep and eat well. I have seen my doctor about this and he prescribed antiobiotics. I am concerned that it occasionally happens-is this a sign of lung repairing process? Should I have a phlem analysis? Thanks to which ever doctor answers this cause it has me worried!