it will be valuable to know about history of recurrent episodes of upper respiratory tract
infections. If the answer is affirmative such blood may be due to it only. From history it is apparent that there is only one episode. start being evaluated for respiratory conditions like asthma
, allergic rhinitis
etc. chest x ray
may be normal in many chest conditions including of yours. get your Pulmonary function test done and follow up with a chest physician. If you smoke, the best you can do quit it in first instance.