What might cause green poop? my son is 7 1/2 weeks old and has only had breastmilk, either from the breast or a bottle. He has been having it for about the last 2 days. He was very fussy for about 3 hours this afternoon, i'm not sure if that makes a difference. He did have a yellow poop diaper in between the green ones. I just have no idea if there is something wrong or if this is ok.