Thank you for writing in,
can be due to many reasons.
Some may be physiological like in the early years of menarche
or in the last years just before menopause
It can also be due to pathological causes like PCOD, thyroid disorders, hormonal
You need to get a thorough gynaecological evaluation to find out the cause behind it and treat the cause to correct it.
Kindly visit your gynaecologist and get it diagnosed and take appropriate treatment.