Small cracker crumb size speck of bright blood in stool only, painful 2 pass happened 1 other time in teen no diaherra. cause? cancer??

Probably Hemorrhoid: Cancer is very unlikely. If you have a history of constipation, you may have developed a hemorrhoid. Generally, people will notice a speck (or sometimes more) of bright red blood on the toilet paper after a hard bowel movement. If there is blood in the stool (red or black/tarry which is older blood) or a substantial amount of blood, you should see a doctor to rule out other causes.