Does your dog ever have nightmares? I have two…

Does your dog ever have nightmares? I have two 8-month old German Shepherds, and one of them has been whimpering/crying in her sleep recently. My Lab, who I've had for 14 years, has never done this. To my knowledge, the GS has not experienced any recent trauma that might cause nightmares. Thanks in advance for any help you can give!

I have not experienced much of this. My dogs occasionally grumble or woof in their sleep but I can’t recall any whimpering or crying. Has it happened more than once, or quite often? 

Maybe some of my followers are able to help?