My daughter Fenix (funny,huh?) had some blood in her stool back in Feb. She also had a very loose stool, like diarrhea, so I took her to the ped. They hospitalized her, thinking she had some sort of infectious disease. I really thought it was just an anal fissure, due to her sometimes explosive stools and gas. They seemed to think it was either from some disease, bleeding nipples (they checked my nipples), a bowel obstruction, or an allergy to dairy products in my diet. After three days in the hospital,and all kinds of tests and x-rays (they almost did a colonoscopy,too), they decided it was...an anal fissure. It healed after about a week, and hasn't been a problem since then.
All that just to say that it might be a real minor thing,and I wouldn't worry too much about it, unless it continues or changes color or something.