Here is a link about lactulose
http://en.wikipedia.org/wiki/Lactulose
Don't panic however, as although it is often used for dogs with liver problems yours may not have a liver problem. As I mentioned before, given her age, and knowing what I have seen sometimes in puppies before, she could very well outgrow the problem. It could be with the intestine itself not quite working properly. It may correct as she gets older and if it stays the same diet management and little lactulose occasionally may do the trick.