There is an interesting article about this on Grantland.
Things That Are Bothering You, Got You All Hibbeldy-Jibbeldy, or just downright pissed, RIGHT NOW! - Page 3728
Srs answer - fresh, red blood in your stool, with no other symptoms, could mean that you have anal fissures or hemorrhoids. Neither will kill you, but you should have it looked at if it happens again. If it was a one time event then it was probably an anal fissure, especially if you've been constipated lately. On the other hand, black, tarry stools are a sign of bleeding higher up in the GI tract and can be very serious. Go straight to your MD if you see this.
BTW, IANAD, and my advice should be treated as the uninformed comment from a stranger on the internet that it is.
On the other other hand, this: