[k0k0]

Wrong. There is melena, which is a generally unmistakable consistency of black sticky stool (nothing anyone would identify as "blood") that is indicative of upper GI bleeding - not the colon. It doesn't really indicate severity because it may be fast or relatively slow. Both upper and lower GI bleeding can be serious in terms of acuity or seriousness of disease (cancer).

However if the internal bleeding from the upper GI system is brisk/fast enough it will come out bright red. This can happen to people with esophageal varices (sequela of liver disease and cirrhosis) or brisk stomach or duodenal ulcers and the prognosis is usually poor, but very likely they will be getting emergent endoscopies and/or mass transfusion if they make it out of the ED.

You can't tell by color.

> That may be an internal bleeding.

Colonic bleeding is just as internal and will typically be red or maroon.

[thinkingemote]

Just a little aside for those doing a search in the future: many things you consume can also turn poop reddish so be sure to think about what you had for dinner if you've just noticed it. For example; beetroot, red wine, berries, stuff with food colouring etc.

[joveian]

In particular what you ate the past sveral days since it takes time to go through (exactly how long depends on your diet and body, but I think 2-3 days is common and a another couple of days is not uncommon). It takes long enough that it is easy to forget.