What Causes Delay In Menstruation Other Than Pregnancy?

i am 31 year old..my last pregnancy was in july 2007 .i had a normal delivery..sibce then i didnt have a normal periods.i had a D and C done last april...since then i had a regular periods that is 60 days cycle./but from the past 6 months i had a regular 28 days cycle..i had my L.M.P. DEC 21ST.i didnt get my periods jan 21st..i took meprate 10mg on the 9th of february and i didnt get a period till date ..what should i do?should i wait or do i go in for another medication as i got negative result in urine preganacy test