Sounds like floppy tail syndrome. This can often be caused by over-exertion or cold baths.
Its indicated by the top few inches being perfectly fine but the rest of the til droops.
Personally, if its not painful to touch, I wouldnt bother with a vet visit until youve given it a couple of days.
Barney had this about 6 months ago, he had a cold bath and then a massive run on the bike and then his tail went floppy except for the first 2 or 3 inches. I panicked at first but after googling, I decided to give it a couple of days and within 48 hours it was completely back to normal.
Edit to add; See I even started a thread about it - which noone rpelied in http://www.dogforum.com.au/dog-healt...-syndrome.html