It could be because you have a cheast infection. You must go see you doctor and get it sorted.
Is this blood in your sputum a 1 time event or have you had more episodes since? Tell me a little bit more about what is going on.
If i understand correctly, you have been smoking for 3 years? How many packs per day?
Do you have a fever?
Describe your cough a little more.
Have you been coughing a lot? It could be a number of things (i.e. dry air, excessive coughing or you may have an infection). If you are alarmed by it, maybe you should go get it looked at. Was it all blood or blood tinged?