
To scan at 23 days is too early, even folk who scan different species as a job wont scan a dog before 28days and prefer later. There would be no outward signs that early either so think you need to go back to another vet practice.
I had a bitch have 4 phantoms and the shortest was 5 weeks and the longest 7weeks, she looked more pregnant than a bitch who had been mated 3 days before her, increased girth and weight, then over night would return to normal.
I think you can be sure that this late and several scans later she is not pregnant.