With symptoms starting and ending at the vets office, sounds like stress to me!! A staff change could definatly be the problem. A new person could be less gentle, make more noise, any number of things. Viral and bacterial things don't incubate fast enough to start a bloody diarrhea in 30 minutes.
However, the vet did not handle things very well from the sounds of it. Since she was not very nice when you tried to get more info I think you are right to go elsewhere.
That's my 2 cents.
Chrysta