It will be interesting to see what Sqad say but my (totally uneducated) guess is sciatica, brought on by a herniated (= slipped) disc.
The usual treatment is nothing more than over-the-counter painkillers, plus a combination of rest and gentle exercise, with the condition gradually improving as time goes on. However, if the pain is bad (or the problem is taking longer to clear up than usual), a GP might try something else, such as a steroid injection.
Whatever the cause though, I hope that you'll be fully fit and dancing the can-can again soon ;-)