Might be a sprain, but it sounds like it might be a classic case of "cold tail" to me. It's a condition (not serious) very common to Labs. Impacted anal glands is one of the causes. My Lab Star had it once. After a day or so, she was fine. Hoping your girl's feeling better! Here's a brief description...

Cold Tail
Also called "wash tail" and "limber tail", "cold tail" occurs when your dog's tail goes limp and he bites at it as if it were a foreign body attached to him. This condition is not serious and should go away in two or three days. It seems to be associated with swimming in cold water (hence the name). It's thought to be a reaction on the part of one of the glands at the base of the tail, or perhaps a sort of muscle spasm. M. Christine Zink covers the condition in Peak Performance; it is not typically listed in veterinary handbooks.