[smartpuppiepets]

First, welcome .
Wow, this sure is a mess isn't it?
The changes of diet, stress , and all other factors can contribute to the stools problems . The fact that she is a pet store puppy does not help it either but it sounds like you did not know any better before getting her. You need to read more and catch up on a lot of good information here.
It may be other parasites, such as coccidia or gyardia , these also cause mucous diarrhea and may not show up on a regular stool sample. Unfortunately your best bet may be taking her back to the vets once again since you may endanger your puppy trying to treat her at home. If she is dehydrated , her skin will be "tacky" and so will be her gums. If she is dehydrated she will need not only oral liquids , but she may need some injected fluids or subcutaneous as well.
But one thing that concerns me a lot is the fact that you are a very inexperienced owner and puppies dehydrate and decline so fast .
Please consider taking her back to the vet, or to another vet from a hospital that can run more tests and give you a sure answer.
I wish you all the best. I know other members will be answering here and you will learn a lot from being a part of YT .
XOXO