In general, bright red blood means a tear or some kind of problem near the end of the line. Dark red or black blood means something further up the tract, possibly intestinal. Any blood in the stool is cause for concern, but there are things you can and should rule out before you need to call the doctor.
Not eating enough fiber or drinking enough water can make your stools hard, causing tiny rips, cuts and tears in your lower colon when you go to the bathroom. The smallest little cut can bleed quite a bit - so it may be a teeny problem that looks massive. If you've had a significant change in your diet or if you've noticed a sudden change in your bowel movements, then do as some others suggested and get some more fiber and water. If the bleeding continues past a couple of days, call a doctor.
Any dark blood in the stool is a reason to call your doctor. My brother had dark blood in his stool that largely went unnoticed because he didn't know what it was. When his doctors tested him for surgery on his knee, they found he was badly anemic; the source of the anemia was traced to bleeding polyps in his colon. He was finally diagnosed with Gardner's Disease and had his entire colon and part of his lower intestine removed. Biopsies later revealed those polyps were all pre-cancerous.
I'm not trying to scare you - chances are, from what you've described, it's the first case and you just need to make some dietary adjustments. But keep an eye on it and if you still see it in a few days, call the doctor. Better to be safe than sorry.