It sounds like Rotavirus which is a more severe form of gastro. Both my boys had it when they were young and the poo you are describing is the same they had. It also had a really off smell rather than a normal poo smell if that makes sense. The only way to know is to get a poo sample and have it tested, which is what I did and it came back positive.
Rotavirus is now immunised against as it was becoming very prominent and very serious in young babies.
The advice I was given was very little food, lots of liquid (hydrolite icepoles) and water and BF . Have you also tried Nurse on Call 1300 60 60 24

I hope she is feeling better soon.