Patbee here.
Thanks to you all for your replies re 'Lab with sudden pain'
Just done a search on 'wet tail syndrome' and it so sounds like what Meg has !! http://www.working-retriever.com/library/cwtail.html
Cold water swimming, excessive tail wagging, confined for a while after exercise (journey home from beach - 2hrs) and not used to this amount of exercise in one go.
Dare I print it out and pass it on to the vet? Do you think she may be offended?