Hemorrhoids are swollen veins in the lowest part of your rectum and anus. Sometimes the walls of these blood vessels stretch so thin that the veins bulge and get irritated, especially when you have a bowel movement.
Hemorrhoids are one of the most common causes of rectal bleeding. They're rarely dangerous and usually clear up in a couple of weeks. But you should see your doctor to make sure it's not a more serious condition. He can also remove hemorrhoids that won't go away or are very painful.