Why Am Coughing Up Blood?

Recently, (since Wednesday) I have been sick: sore throat, headache and coughing up phlegm. Which usually means a sinus infection; not a big deal. But I am coughing up blood along with the phlegm, (Since Thursday) and I am getting sort of scared now. What are conditions that can make you cough up blood and how serious are they? Also I'm young, in my teens