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

Lots of reasons: At your age perimenopause is the most likely answer. Pregnancy, stress, dysfunctional uterine bleeding, annovulatory cycles 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.