Call your doctor. Sometimes babies develop blood in the stool due to intolerance of milk protein which is easily treated by removing milk proteins from mom's and baby's diet. Blood in the stool can also be caused by a small tear in the anus. Your doctor must assess your baby, however, to determine whether the cause is one of these two, or something else.
Contact your doctor. You baby may be having a milk or food allergy. The most common foods to cause this problem are milk protein, soy, eggs, shellfish, and nuts. In addition, if your baby has fever or is acting sick, your baby could have an infection. In general, persistent blood in stools is not normal in babies and should be evaluated best by your pediatrician.