First of all, it is extremely uncommon for a person your age to develop breast cancer. These discharges are most likely be due to hormonal changes, which in a teenager, can be erratic and fluctuate throughout a menstrual cycle. The presence of infection is also a possibility, especially if accompanied by swelling, and the discharge is foul smelling. If this is the case, a course of antibiotics may be warranted. As you have pre-empted, it is still best to have this checked by your doctor.
Regards and God bless.