If it was lactose overload it would be watery/frothy as well. Does she have much wind? I know that green poo often goes with wind. When I was about 4 or 5 I remember having bright green poo (not sure if my memory is accurate as to the colour, but it was definitely green). I told my mum and she just said it meant I had wind. I was otherwise well so she wasn't concerned.

If DD seems well, I wouldn't worry too much. It might be out of her system now. But a trip to your GP or MCHN won't hurt if you want to put your mind at rest.