When you went to your doctor, did he/she say you have rales in your lungs? Did he say that your lungs sound clear? Judging by what you said, the mucus it coming from your nose and not your lungs. If you had lung cancer, your white blood count would be high, and many times the mucus from a cancerous lung would show spots of blood.
I am sure that you have allergies, or bronchitis and the antibiotics will clear the bronchitis if that is what you have.