I am 19 years old, and my period is 15 days late. how likely is it that i am pregnant? or is there something else that could be causing this?

Many possibles: There are many reasons you can skip a period in a particular month, but the number one cause is pregnancy if you are sexually active and not using reliable birth control. It's easy enough to get a home pregnancy test which would be very reliable if you follow the directions if you are already 2 weeks late. Other than pregnancy, change in routine, diet, illness, stress can cause a missed period.