2-3 years at your age is a long time. I don't mean to offend, but are you really skinny? When I was younger and running, I basically had zero body fat and did not get my period; something about producing hormones needing some body fat. However, if I stopped running for a couple of months, I would get my periods. I'd start running again, and the periods stopped.

Anyway, you should go see a doctor. The exam is over with quickly.