I'm assuming that this isn't menstrual-related. Basically, blood in the stool is never a good sign. If you tell a doctor you have blood in your stool he will definitely want to examine you, possibly do a colonoscopy. It may be something very serious, so you should see a doctor about it. There is also a possibility it could be internal hemmorhoids, so don't freak out about it, but definitely tell your doctor.