My rule of thumb...or rules
:
I never let a dog go longer than 24 hours without eating. That is the first sign something is wrong. I take to the vet immediately.
I never change their food before they are seen by the vet and diagnosed. Food changes *can* add to the problem. Also, you never know for sure what has *fixed* things if you change up.
Once the vet has seen and diagnosed, I follow the vet instructions. If they think a diet change is warranted, I usually go with what they recommend which is usually Hill's ID.
Good luck tomorrow! There are SO many GI things...I would not even try to guess what it is. Let us know what happens, please.