What Causes Coughing Up Large Clots Of Blood After Treated Tumor?

hi there my dad has just undergone surgery in india to remove a rapidly growing tumor at the base of his tongue. The surgery was a success however he is having complications. Lately he has started coughing up huge clots of thick blood. He even put a spoon down his throat to remove the larger clots. We rushed him to hospital and is on morphine at the moment while the doctors are trying to find out where the blood originated from. Could you please share some light on the situation , just to ease our nerves. Thanks for your time. Much appreciated