I would not have ultrasounded at this stage for one of my own girls though I might have asked the vet to run a relaxin blood test if I was really anxious about the pregnancy.
Usually you can tell a dog is pregnant as you see a clear sticky discharge from the vulva starting around weeks 4-6.
She has been eating the dog version of 'candy' with highly tasty meat and high fat peanut butter from what you said.
Both of those could cause her pancreas to overproduce enzymes making her uncomfortable so its better to feed a good dog food.
As you have the prescription food on hand (and Science Diet is not a favorite of any of my dogs either if that is the one). Try a little peanut butter 'sauce' for flavoring and see if she likes it that way.
The way mother nature works is the pups get everything nutritionally first so if mom isn't eating the pups will still get nutrition and mom might lose any fat she has on her body.
But up to this stage they really have only been dividing cells and becoming more complicated without getting a lot bigger.
Bear in mind when consulting with your vet that, unless they are a specialist in reproduction, pregnancy really isn't anything they study much in school though they do train in c-sections.
I usually x-ray around day 54 to see how many pups are coming and to ensure none are too big to be delivered without vet help.
On this page if you put in the date of the first breeding you should get little buttons to one side that you can hover the pointer over to see what goes on in that week of pregnancy