What happends if you haven't gotten your period 3 weeks after unprotected sexual activity?

Lots of options. At your age pregnancy is possible. Premature menopause, stress, dysfunctional uterine bleeding, annovulatory cycles, hormone problems and many other things are also possible. If you are concerned, it is time to call your family doctor! until you know for sure, just to be safe, don't to anything that would hurt your baby if you are pregnant.