Not necessarily true. I've found this differentiation to be highly unreliable in the emergency department.
Blood on toilet tissue, but not in the toilet, usually is a sign of internal hemorrhoids, but not always. Some people have other sources that begin that way, and then progress to a loss of a liter or more of blood. I just saw such a patient only two days ago -- she bleed from an aortic-enteric fistula. Apparently she had a sentinel bleed at home, came in complaining of abdominal pain, and I only CT scanned her because she was complaining of abdominal pain and had a family history of aortic aneurysms. I was lucky I did. She went to the OR and was alive when I last checked.
You should see a physician to make sure you do not have a serious medical condition. If it's bright or dark, it's usually (not always) from a lower source -- below the duodenum. If it's black, it's usually from above the duodenum (i.e., stomach) because the hemoglobin reacts with acid in the stomach. However, very brisk upper GI bleeding can present as dark or bright bloody bowel movements if the transit time is fast enough.
Some causes of lower GI bleeding are:
- ulcerative colitis/Crohn's disease (especially with abdominal pain, diarrhea, and mucous in your stools)
- diverticulosis
- internal hemorrhoids
- arterial-venous malformations
- aortic-enteric fistulae
- polyps/malignancy (usually doesn't present as large amounts of blood)
Of course this list isn't complete, but lists the most common causes. I would urge you to see a physician. He or she will most likely want to do a colonoscopy on you to diagnose what's wrong.