Variable Bleeding During Menstrual Cycle. What Is The Reason?

hi am 21 yrs old .. am facing severe problems with my periods .. i have lot of variations in my mensturation cycle.. some times i have heavy bleeding .., sometimes il have very low ,, if i have period in a month wid heavy bleeding 4 next month i dnt have a period at all .. and even if i get so .. it wont b so heavy r so lite normal nly 4 3 days r smthng lyk dat n .sometimes il not hav periods at all 4 3 r 4 months.. i dont knw wats the reason for this .. i consulted my doctor she said it happens in somebdy .. but i jus want to know the reason.. and am absolutly fine wen am having periods r wen i dnt hav a period .. n also plz suggest me can i use sysron-n to get regular periods ..i.e, monthly normal periods do this tab vl solve my problem to get periods soon n clearly often monthly..?