The presence of a small amount of blood in the mucus you spit-up is abnormal, but not necessarily an indication of serious disease, especially in an otherwise healthy 16 year old. In most instances, such blood originates in the nose, may then drain into the throat and is then spit-up. Or it may originate in the gums.
The most important question is whether the bleeding does or does not recur. If it does not recur and you have no other symptoms, you can probably forget about it. But, if it recurs, even just one time, you should arrange for a nose and throat examination, preferably by an ENT specialist and get a chest x-ray.
Good luck.