Oreo is also a lab, and twice she has had a sprained tail after too much exertion swimming. Each time, I noticed that she didn't wag and looked "sad"--but then I realized it was because her tail was down and not up. She could wag the end, but she couldn't lift it or hold it up like normal; I could tell that it was sore.

She went to the vet, and he ruled out the many other things that could cause this. He said that she had sprained it about 2 inches down from the base.

Like the other people before me have said, there are many things that could cause a tail-down dog, from a minor problem to major injury to emotional stress. I would suggest taking her to the vet if it isn't better soon just to be on the safe side.

Has she done any new activities where she could have strained her tail? I hope she feels better soon.