What Causes Change In Stool Consistency Of An Infant?

5 week old baby has 2 poopy nappies since last friday. Today she filled a nappy with khaki green hard poop. Shes formula fed. She normally had 3 to 4 per day until last friday. Nothing has changed regarding feeds. Ive started giving her cool boiled water and she serms to be suffering with colic. She is on antibiotics for 3 months for uti