What are you feeding her? Like Gemma said it could be a lack of good nutrition but it could also be one of those things, some dogs do it for no apparent reason.

Why isn't she toilet trained? I would definatly start there.

Prevention is always better than a cure so try to not put her in a position where she can eat it if at all possible. Also you can use treats to distract her from eating her mess while you clean it up.