What is she taking the previcox for?
She may be reacting to the medication, but it could be something unrelated. The combination of vomiting and diarrhea can quickly dehydrate a dog--this can be very dangerous. Not eating, in and of itself, indicates a trip to the vet.
I think your girl needs to see the vet as soon as possible. Can you get in touch with a vet by phone to get some advice till you can get an appointment? I have no idea what the side-effects of previcox are in dogs, but at the very least, you should discuss discontinuing the meds till she sees a vet.
Good luck with your rotty! I hope she's feeling better soon!
And welcome to the board, hotkey13.