What you describe sounds like "Limber Tail". There is no absolute reason known to cause it, however, it is believed to be muscle strain from the dog using the tail more vigorously than normal. A lot of veternarians are not familiar with it. Working breeds seem to be more susceptible. I'm sure if you do a search on the net under Limber tail or Frozen tail you will find a few hits.

Dixie has had this several times and Dusty has had it once. Both of them usually got it after vigorous workouts and swimming in colder than normal temperatures. It takes one to two weeks to completely heal (depending on how bad it is). Rest is recommended for the first few days and after that miminal exercise until the tail starts to freely wag again.

There is swelling and pain associated with this condition. Depending on how severe it is you may want to consider taking them to the vet for a anti-inflammatory and pain shot. Dusty's was mild and he didn't require a shot. I just gave him some coated aspirin for a day or two. Dixie's was a lot more severe so she got the cortisone shot.

I hope this was informational and Finn is doing better soon.