Hi, and thanks for your medical question on eHealth forum!
To confirm you can take a beta HCG test in blood. This is usually repeated after a week. A rising titer confirms pregnancy.
If this is not the case, then other causes will have to be looked into. Stress and anxiety is a major factor. Thyroid problems and abnormal levels of FSH, LH, prolactin, androgen hormones or male hormones and estrogen hormones too can be the cause. MRI of brain may be required to rule out pituitary tumors secreting prolactin. Another major contributor is polycystic ovarian syndrome (PCOS). This can be diagnosed by a CT scan or abdominal ultrasound and tests for insulin resistance. Smoking is also often associated with secondary amenorrhea and can be worse in presence of stress. Sometimes tumors of the ovaries and brain may also cause absence of menstruation. Severe anemia and malnutrition, excess weight gain or weight loss, hypothyroidism too can contribute. If you have been on pills or depot shots then this can be post pill amenorrhea.
Since a confirmed diagnosis cannot be given on net please consult your gynecologist regarding this. Hope this helps. Take care!