A nosebleed is when the tissue lining your nose loses blood. It usually occurs in 1 nostril only. Most nosebleeds occur because of minor irritations or colds, nose picking when mucus in the nose is dry, injuries like a broken nose, allergies, infections, high blood pressure or blood clotting disorders. For some people, taking aspirin and other drugs that cause the blood to thin can make nosebleeds more common.
Attribution: Straight Talk Foundation