Dear Wendy,
I don't think colitis can be diagnosed over the phone, and unless they took poo sanples, they shouldn't be prescribing anitbiotics really ...
Even if there has been a local outbreak, they should treat each patient as an individual with possibly different symptoms ... it could be anything really ...
Many Vets obviously think that antibiotics are the answer to all the outbreaks of gastric problems that are going around .. as advised, I would change your Vet, however difficult that may seem, since they are obviously not bothering to do a proper diagnosis but rather placing their bet on something else that is going around ...