Lack of menstruation (amenorrhea) can be caused for many reasons. The first thing doctors consider when amenorrhea occurs in women within a healthy reproductive age is pregnancy. Pregnancy can be easily excluded or confirmed with a pregnancy test. In your case, pregnancy has been excluded because 2 pregnancy tests are negative.
Another very common reason for amenorrhea at your age is the presence of functional ovarian cyst. Functional ovarian cysts produce hormones (estrogen or progesterone) and are not controlled. Excessive production of such hormones can delay menstruation. Functional cysts can’t produce hormones forever because they usually degenerate after 2-3 months. Then, a period will occur.
More rare and infrequent causes of amenorrhea could be also include: hypogonadism, hormonally active tumor, taking off birth control pills, early menopause, polycystic ovarian syndrome (PCOS)…
You can go to your gynecologist for examinations to identify the cause of amenorrhea. The examinations should include a vaginal examination, Pap-test, vaginal ultrasound and hormonal analyses (if necessary).