you are best off seeing a doctor as we cannot diagnose but give advice only.
bleeding from the back passage could be either haemorroids, crohns disease, or even torn tissue if you are straining too much.
if you are constipated drink plenty orange juice this will sort it, if there's dark blood in your poo then definately see your doctor thats not a good sign.
if you loose any more blood you may become anaemic and need iron please get it checked better safe thats sorry
If you are straining yourself, or constipated, it could produce a little blood when you have a bowel movement, but there shouldn't be a lot of blood like that. It could also be hemorroids, do you know if you have them? Sometimes they can bleed. I would suggest you go to the doctor though, it sounds from the way you describe it, that you have a lot of blood and that can't be good.