The most common cause of blood in infant's stool is either a fissure (anal tear) from baby straining with the passage of the stool, or a crack nipple from the mom. I wouldn't be too worried about it if the baby is eating well an sleeping well. But if conditions persist for days, you should get it checked.
My little boy had blood in his stool at 1 month old, it was smelly and watery as well, went to CHEO and waited for 8 hrs, never heard back from them about results...
