Take it to a vet?

It's hard to know without seeing it, but it sounds like you have a kitten with a kink in its tail, a common thing. A kink is a knotty bump in a spot on the tail bone, and can be visible or not, can result in a visible bend in the tail, or may only be felt. Kinks in tails are particularly common if your kitten has any Siamese in him/her. In Thailand, a cat with a kink in its tail is considered lucky. Also, while nowadays the Siamese kinked tail has become a show "fault", in the early shows it was mandatory for a Siamese to have a kink in its tail to be considered a true Siamese.

Here is a thread where I posed a question about a kitten with a kink in her tail.

Here is a link that might be useful: Thread about Kink in Kitten's Tail

It's a type of deformity, but I don't think it's worth worrying about at this time. The site below may help you feel better.

Here is a link that might be useful: curly