For the first time after peeing one dark drop of blood came out of my penis. straight after i urinated. could you list a few reason why the hapend ?

Blood in urine: Blood appearing from the penis at this time of urination is a frightening experience. Blood emanating from the penis immediately after urination likely originates from the bladder, prostate or the urethra. While many causes of such bleeding are benign (non-cancerous) such as infection or prostate enlargement, patients with blood in the urine need urologic evaluation to assure no cancer is present.