I spit out blood right after smoking a cigarette?

Be seen.: Blood is abnormal and can come from a variety of areas. You would need an oral/ENT exam, chest x-ray and referral to a lung doctor, if I was seeing you, unless I thought you had an infection. In that case I would do the exam, get the x-ray and treat with the antibiotic and see what happened.

That's bad.: Generally speaking, we doctors like when blood stays inside the body. When a smoker has an episode of bleeding like this, despite your young age,,it deserves an investigation into cause. You should be seen by ENT and Pulmonologist right away.