Do you have a fissure in your rectum? When my son was very small, milk made his stools like bricks and the poor little guy developed a fissure and he tried to keep his BMs inside -- then we found out why. So, if you have been constipated before, you may have something like this.
Also, if you been really annoyed or angry the day before, you may have a hemorrhoid (sp?) that is acting up.
Or, it is something else. Best of luck in fixing this -- much discomfort! :(