What causes coughing of blood during needle biopsy of lung?

I am not a doctor or medical expert. You should ask your doctor these questions, because operations vary by the patient and hospital.

I don't know what causes it, by I did have a lung needle biopsy today. They put me completely under (perhaps it's because I'm only 15) so I don't know if I coughed up blood during the operation. Coughing up blood is what got me to go to the walk-in in the first place, where the found a mass in the upper lobe of my right lung.

For my operation, they inserted the needle through my back. I am now sore, but all is well. My doctor(s) told me that coughing blood was normal and that if they would decide to not put me "under", that I should not worry if I did cough blood.