You should be certain. If he wanted to bite, he would. His snap was a warning. He was uncomfortable in that situation and he reacted in that moment. This doesn't mean he will do this again.

If the vet says everything is ok, give him a nice place to sleep, somewhere more secluded or in a crate (with the door open). Or try to discover what happened, if she pushed him or there was a noise what scared him.

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Festus & Anca
“No culture can live, if it attempts to be exclusive.” - Mahatma Gandhi
Don't assume...ask

I agree with the pain thing - deffo worth checking out - rotties are known to be very stoical when it comes to pain and hide it well.

Also, I would also be wondering about the whole waking up quickly thing, maybe he was just dreaming and woke up quickly as others have said.

Just as a extra precaution, perhaps make a point of doing some extra obedience, just to re-inforce his good behaviour etc.

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Fere libenter hormines id quod volunt credunt