Seems like maybe an allergy to something in the cage? But a lot of rat owners would put their rat on antibiotics in this situation... my attitude is, better safe than sorry. Generally, my advice is just listen to the vet and see how s/he talks about rats. I recently moved and had the local vet and his staff explain to me that they're 'not allowed' to have the rodent owners present when a rat is euthanized, while at the same time they do it when it's a cat or a dog... the policy reflected something important about this place, which is that they really don't know much about rats and treat them as second class citizens. Just make sure that the vet respects your animals and listens to your concerns. Good luck!