Being that she is only 8 weeks, she is probably really tiny, and she can go down hills fast.
TJ was 13 weeks when I brought him home, and within a week he started to have these gel poop episodes. He tummy would make crazy noises too. I was at the Vet like every 3 days.
I don't know what is wrong with your baby, but don't be afraid to go back to the Vet or find a new one
I bought Children's Pedialite and I would force with a syringe that to TJ to make sure he stayed hydrated.