This blood is surely not a sign of lung cancer, given your age, your trivial smoking history and the normal chest x-ray and CT scan. Blood in one's phlegm should never be ignored but you should not need any further testing beyond the imaging you have already had.
And, it should not be assumed that the "speckle of blood" came from your lungs. The normal CT and x-ray suggest it didn't. It is just as likely to have originated in your nose, throat or sinuses. If it occurs again, you should see an ENT specialist, who can examine your upper airways directly with a fiberoptic laryngoscope. He/she may want to do a CT scan of your sinuses, to be confidently re-assuring.
There is one really remote, rare, benign possibility. Make note if there is any relationship between the appearance of blood in your mucus, and your menstrual period. If so, share this information with your doctor.
- Just to clarify this symptom is not recurrent - (every 2months) I experienced this yesterday, June and once maybe a few years before that.