What Causes Delay In Menstruation Other Than Pregnancy?

i had my menstruation on the 25 July 2010 for 5 days and again on the 29 August 2010 which was 37 days cycle for the first time. i had unprotected sex on the 11 September 2010 which was 14 days of the cylce and i did not have my period for September until today the 07th October 2010. i dont have any sign of pregnancy. i am fien. so whats wrong , why did i miss my preiod?