I am very glad to hear you are working closely with your vet. Personally I do not see any reason to have her x-rayed so soon. If you do it to early then you may not know for sure how many she is carrying. I think most people wait longer so as not to expose the momma and the pups to early. Here is a good website that explains a bit about xray and what to expect.
G9923 Pregnancy Determination in the Bitch | University of Missouri Extension
The nipples don't usually start getting bigger until more toward the end of the pregnancy. Each female is different. I have had some females with really tiny nipples and some with huge nipples.
How many times was she bred and over how long of a period? If she was bred several times then one of the later breeding could of taken and she might not be as far along as you think. Another reason to wait a little longer on the x-ray.