My DD had this a few weeks ago (sudden change in bowel movements with no temp etc). Turned out to be an unspecified gastro bug (they couldn't find the bug responsible in a poo sample) which then became secondary lactose intolerance. She is fine now - back to her old poo habits minus the thick, dark green sludge consistency and we have just begun re-introducing formula containing lactose and had no problems so far (touch wood!). I hope your DD doesn't have anything too serious and recovers well.

MG