He seems fine today. We've just got back from a 2 hour hike through the woods, and he's snoozing in his bag. We had an earthquake here a few weeks ago, and I did wonder if he was hearing/sensing something like that. He was definitely frightened
Other than that, one of mine does this when she hears noises - the worst being a smoke detector beeping when the battery is flat, but also when the bastarding hunters are in the field behind my house, fireworks etc
Out of the blue, Jas just started shivering and tucking his tail between his legs. DS was stroking him at the time, but not in a scary way (he's 12). Obviously, DS stopped and called me. The dog got up on the sofa next to me, burrowed under my arm and shook like a leaf for 20 minutes. I have no idea why. He has now taken himself off to hide under the dining table. No obvious illness, gums a good colour, no vomiting or diarrhea...Any ideas?