According to 3 negative pregnancy tests, it doesn’t seem likely that you are pregnant. Nausea and cramps don’t suggest pregnancy. According to your reproductive age (18) you may be experiencing a hormonally active ovarian cyst (functional cyst) that has delayed the menstrual period. Functional cysts usually exist for 2-3 months and can cause such a delay. Functional ovarian cysts can be detected with ultrasound. It seems that a gynecological examination at your gynecologist's office would be more beneficial to you than another pregnancy test at this time.