My period was supposed to start today but it hasn't. I'm experiencing some PMS like breast tenderness but it started like 2-3 days ago (which seems a bit late) and I have had some light cramping/twinges as well. It is now 8:31 pm and my period has not started. I'm a little worried but I really shouldn't be, I'm 17 years old and I have never ever had any type of sexual interaction in my life, except for masturbation (but that can't cause anything right?). Why hasn't my period started!?!?