In August 2010, after having the same problem you described (which was a hemorrhoid), I saw a gastroenterologist, who set me up for a colonoscopy. They made me drink 3 liters of foul liquid the day before the procedure to "evacuate" everything. I had the procedure done and $1,400 dollars later, I had a non-serious polyp removed during it, and that was it. Obviously they saw the hemorrhoid in their search too.
Afterwards, it happened on very rare occasion (blood in the stool), but not nearly what it did before. But then again, I took them up on their advice on trying to eat more fiber and taking a fiber laxative for a little while. They informed me that it can start becoming bothersome after a harder than usual bowel movement that causes minor tearing which breaks open the hemorrhoid and releases the blood into your stool.
It happens even more rarely lately, most of the time just a tiny bit one random day and that's all. Just listen to your doctors and they'll get you taken care of.