I wouldn't worry, You might have just pulled something slightly and the muscles are cramping around it.
Go lightly for a day or so and see how it still feels. Though obviously if it gets worse then go see a doc ASAP.
42" jeans(25/01/2013) 40"(28/02/2013) 38"(20/03/2013) 36"(25/05/2013)
Down from 60" waist jeans since June 21st 2012.
Still keeping to my 26" jeans, but they are too tight for comfort. too many tasty things in the US, and over Xmas.
Onwards and Downwards! http://www.menu52.com/