What you describe suggests that you may have smoking induced chronic bronchitis but we should not assume that that is the cause of blood in your sputum, even a small amount of blood. Your history suggests that your lungs, including the bronchial tubes, are very sensitive to the toxic effects of cigarette smoke, especially given that you smoke very little. That raises the question of your having a family predisposition to chronic obstructive pulmonary disease (COPD) or the genetic defect called alpha-1 antitrypsin deficiency. Other possibilities at age 18 would include cystic fibrosis with or without a condition called bronchiectasis.
What you have experienced should be seen as a warning sign that you should quit smoking and never smoke again.
For now, you should definitely see either your family doctor or a lung specialist also known as a pulmonologist, for physical examination, some blood tests, a chest x-ray and culture of your sputum for routine bacteria and for tuberculosis (TB). You should not delay.
Good luck.
I'm 21 and I've been smoking for about 5 years now I noticed the same thing this winter. For me it developed while i had a cold and was taking pseudoephedrine in 90 mg doses, and 1200 mg of gaufenesine and 30 mg dextromethorphan hydrobromide in mucinex tablets. It went away after the cold went away. My only thoughts might be that it was caused by excessive dryness in throat as the above was slightly overkill, but I work in fast food right now so i figured better safe than sorry. I've also noticed it tends to appear on my smoke breaks when it is really cold and dry outside. My only solution was to cover my mouth when it was really bad out and sometimes run a humidifier in my bedroom and that worked pretty well.