It can be due to secondary amenorrhea. Secondary amenorrhea is when a woman who has had normal menstrual cycles stops getting her monthly period for 3 or more months.
Causes of secondary amenorrhea include stress and anxiety, malnutrition, depression, overexercising, sudden weight loss or gain (obesity),thyroid disorders and hormonal imbalance due to polycystic ovarian syndrome(PCOS).Smoking is also associated with secondary amenorrhea. Sometimes tumors of the ovaries and brain may also cause absence of menstruation.
Some of these causes are modifiable like if you are having stress then yoga and meditation and antianxiety medicines can help you out. Eat regular healthy, nutritious food and indulge in moderate exercises. If still the symptoms do not improve, then consultation from a doctor is required to rule out PCOS or thyroid disorder.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.