Lack of a menstrual period (amenorrhea) is the first serious sign of pregnancy. However, amenorrhea is also present during other conditions such as : functional ovarian cyst, hypogonadism, menopause, anorexia etc.... so it's best not to jump to conclusions. A negative pregnancy test can exclude pregnancy as a possible cause for amenorrhea.

According to your age (ealry 20's), you might be experiencing a hormonally active (functional) ovarian cyst. Ovarian cysts are diagnosed with ultrasound examinations. Functional cysts usually disappear spontaneously after 2-3 months and then the menstrual period will come and the normal cycle will resume.

You can consult your gynecologist about this problem and ask for diagnostic tests and monitoring of the condition.