You need to see a doctor immediately. You should not bleed when you have bowel movements, unless you know, for certain, that you have hemmroids, and even then, they should never bleed heavily, such as you describe. The should only produce a little blood on the toilet paper. If the blood was bright red, then you may have had a hemmroid rupture, which is no big thing, but you need to be checked by a doctor and make sure that's all it was. Otherwise bleeding might indicate a fissure or other tear around the rectum, or maybe one of the IBDs, like UC or Crohn's (D and severe pain being a common symptom of both).