At age of 19 periods are known to be irregular. It can get affected by many things which may cause emotional disturbance like exams, stress of family feuds, relationship issues. But it could also be due to problems like excess weight, PCOD,
thyroid disturbances, etc
Sexual activity can also delay menses by means of pregnancy.
Next Steps
If you are sexually active, do a urine
pregnancy test, which could rule out a pregnancy. If the test is negative it will show pregnancy is not there. Meet a Gynecologist for evaluation of your problem and get expert help from a nearby gynecologist