Any rectal bleeding should be medically investigated because it is not normal. The causes can be as described as above in Missy 1962's posting but there are also more serious causes. Type in rectal bleeding to google/yahoo to learn more. The medical name for the black/tarry appearance of motions as a result of intestinal bleeding is MELENA. Type this into google/yahoo to learn more. Melena should always cause you to seek medical advice.
The more sinister causes of rectal bleeding tend to occur at age 40+
regards
Morecambe
Probably the most common reason for red blood during and after a BM would be hemorrhoids. You could also have a small tear in the rectum, called a fissure. Most of the time when blood is red it is coming from somewhere near the rectum. Blood up in the intestines usually has a black appearance when it passes.