Is this a one off occurrence or has this been happening often? If you notice a succession of watery green poop, your baby may be consuming more foremilk than hindmilk. If baby drinks more hindmilk which has a higher caloric count, your baby's poop will be more yellow than green.
http://www.babycenter.com/404_whats-the-normal-color-of-a-breastfed-babys-bowel-movement_8830.bc
Jessica FD