
Palpation is only reliable if the person has the knack between 23 and 28 days pregnancy, so the vet could easily get it wrong, as it would be past that by now.
Scanning by someone experienced in the procedure is far more reliable especially at around 35 days from likely mating.
I would go to another vet experienced with scanning at 35 days post likely mating date and then get the Alizin jab.
I know people who have been told their bitches were not pregnant only to have pups so would not rely on the vet purely from a palpation.
Alternatively demand an Alizin injection, as better an unnecesray mismate jab (it is a little painful at the injection site and can cause cramps), than an unwanted litter.