Is it possible she ingested something while at the trainer's? What other tests did the vet do? It could be a lot of things, including giardia or SIBO or EPI. Whatever it is, she needs some help immediately, especially if she is lethargic.
Unless she had just had a drink of water, the clear liquid was probably bile. Her stomach is empty and she's probably feeling very nauseous.
I would be concerned too and would take her back to the vet. And the vet should not have given her vaccination when she was ill. That puts more stress on her immune system.
Can you take her to the e-vet tomorrow? Did the vet give you anything for the diarhea?