First illness - help!

[Solo2]

Rollo - golden retriever puppy, 10 mnths old - was acting v different last night - not interested in his food, for the first time in his life - not bouncing around and eager for yet another walk - but going into his crate and lying down. I knew immediately something was wrong and so we took him straight to the vets.

They confirmed that he had a high temperature - she said at the level that humans would just want to lie still and sleep. She gave him an anti-inflammatory injection and we've got to bring him back to the vets in 24 hrs to recheck his temp.

Rollo has a history of gastroenterinal issues but even back then was always bouncy, happy and hungry. This is completely different.

How can he have got so ill? He hasn't 'met' up close another dog for about a week as most of our walks have been where it's rural and isolated and you only occasionally meet other dog walkers. Will he have picked something up from stuff he sniffs on the ground? He always wears a muzzle when we go out, as this stops his gastro problems by preventing him from eating everything and anything. But I presume he can still breathe in and lick things.

Can anyone reassure me? Is this a normal kind of illness for a dog? Does it suggest something much worse?