What Causes Delay In The Periods If Not Pregnant?

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Hi I am shalini, 29 year old, married and having a kid 2 year old. My monthly circle was regular but this time last I got my periods on 10 nov 2010 and till date i havn't got my periods. even I did my pregnancy test at home for 3 times and it was negetive also. yesterday I consulted my doctor and she advised me to take modus 10 twice a day for 6 days and i will get my periods. Please advice me will it work and is it safe also????? thanks