There are 8 Common Stress Signals and this article from www.dorwest.com shows you how to identify them. There are so many reasons that a dog may become stressed, it might be a car ride or even a loud noise.
"In any stressful situation, we can help! Here’s how to help a dog with anxiety and stress:......"
Have a look at their natural calming remedies for your pet, if you have any concerns please consult your Vet..