Missed period!! HEEELP!!

So tomorrow it'll be 3 weeks since I was meant to come on my period.... I'm 17 years old, sexually active, but 99% sure it's not pregnancy. I've been having my period for about 2-3 years and the past year and a half have been pretty consistent, until now. I changed my
diet last June but haven't had any problems with that before. Stress in my life is pretty normal... I just don't understand what it could be!?!? And also don't know when I should visit the doctor about it!! Pleaaaase someone help me !!