This sounds like a self resolving problem called 'cold tail'. It often (but not always) happens with dogs that are working or playing in cold water which is how it got the name.
You can read about this problem here
http://www.geocities.com/ccrhearts/Tail.html
It could be a pinch in the spine area too.
I'd wait and see if it goes away - if it heals up on its own its no worry. If the dog gets worse or seems painful then see the vet.
Hope this helps you!
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