Excessive anal bleeding when I defecate?

Get examined. Any bleeding upon defecation is abnormal. Even occult blood (not obvious to your eyes) is abnormal. Not sure but sounds like you're asking about visible blood. Causes may include hemorrhoids, polyps, diverticulitis, infection, other disorders of the small/large intestine. Eating lots of tomato ad watermelon can also make feces look bright red. Visit your doctor!
Rectal bleeding. Rectal bleeding can be a symptom of many problems. Most often a benign cause such as hemorrhoids or an anal fissure, which is a painful tear in the lining of the anal canal, cause bleeding. A few drops of blood can turn the whole toilet red. However, depending on your health, other more worrisome problems like ulcers, inflammatory bowel diseases and cancers can also cause bleeding. See an md!
Anal bleeding. Excessive anal bleeding always requires a physical examination and evaluation. There are many possible causes of this it is vital that you seek medical attention to determine if this is a serious condition.