If the pulmonary doctor wasn't too concerned chances are it wasn't anything terribly serious. Most serious things that cause blood streaking in the mucus are seen on a chest x-ray (like cancer) so you don't really have to worry about this. It's likely that you cough very hard and rupture a capillary, and this capillary bleeds a tiny amount.
However, if you start noticing a worsening respiratory problem, or more blood in your mucus, or frank blood in your mucus (as in not just teeny streaks or a taste, rather something the size of a clot,) you should definetely see a doctor.
I don't think it'd be a bad idea to keep an eye on it -- mention it to your doctor how often it happens and whatabouts it looks like, like if it only happens when you cough really hard or if you've been exercising very hard or if you're having sinus problems or a bad cold. You *should* ideally be going for a yearly physical. If it isn't causing you any problems and a few chest x-rays (like say 1 a year) show absolutely nothing to be concerned about, I'd not worry about it at all. Some people just have capillaries that are more prone to rupture when they cough forcefully.
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