I have notice on a few occassion while I was brushing my teeth I have cough up blood. At first I thought it was coming from my gums but after doing this a few times I examined my gums and did not find any signs of blood. This do not happen often it only happens occasionally maybe about once or twice every six months or so.
Yes, you should be concerned. Since you coughed up blood only once or twice every six months or so, and you did not mention any other symptoms, the most likely cause is nosebleed that had drained down your throat. High blood pressure is one of the causes of nosebleed, so is cancer of the nasal cavity. For your peace of mind, I suggest that you see an otolaryngologist for a definitive diagnosis.