Dead Tail in Labrador

I hunted my lab in NE Arkansas this weekend; then drove back home to Tennessee. After I unloaded and got on the couch, I noticed him whimpering and his tail in a limp position. He was very uncomfortable.

I’ve done some Google research, and heard of this before. But has anyone else ever experienced this? Plan to rest him this week before hunting again next weekend. If super cold; I will probably leave him at the camp.