I suspect your snake was harboring a pathogen on arrival or picked one up soon after. Perfect husbandry may have prevented the snake from becoming ill--there is no certainty though.
I think the Vet issuing antibiotics was the textbook move. I suspect they do this because this method works some of the time. I don't agree with this procedure-it either fixes the issue or makes things worse. In this case I suspect it will make things worse, but, I am just guessing based on the small information provided.
I would work on establishing a better setup with ideal temps. and realize that this may be a learning experience.