What Causes Absence Of Periods?

Hi. i been wanting to ask, why i havent gotten my period yet and ive went to my doctor to get my depo shot again in aug.3.11 since its been a year since ive been on it. When i got my depo shot in august i had my period on sept.7 -21 usually my periods last only a week but this has happen twice so far and october came and i got a follow up to get it again so i went and got my depo shot. and i would mostly get my period in between which would be nov. but i didnt get it because maybe to the stress that was droped on me, would that be the problem