A mome pregnancy test can sometimes show a false negative but blood pregnancy tests are 100% on target so you are probably not pregnant. According to your age (late teens), lack of menstruation could be due to the presence of a functional ovarian cyst. Functional ovarian cysts usually originate from an ovarian follicle or yellow body. Functional cysts are hormonally active which means that they can produce hormones. The hormones produced by the cyst, in turn, prevent the expulsion of the egg-cell also known as ovulation. The same hormones also act on the endometrium and delay the menstrual period as long as the cyst is active. Functional ovarian cysts are usually active for a couple of months. Ovarian cysts (including functional) can be seen and confirmed with an ultrasound scan. If your period is late for more than 3-4 months you can consult a gynecologist for advice and a more precise diagnosis.