Period is 11 days late????

I'm 17 years old and my period is 11 days late now. I know I can't be pregnant because the last time I had sex was out of my fertile window and we used a condom. We also checked after to make sure the condom hadn't broken. I even took a test yesterday just to make sure and I don't have any other symptoms except a missing period. Could it be due to stress or not eating enough? I workout around 9 times a week because I'm a swimmer and I recently started weight training. Does anyone know why my period could be so late? Thank you for any advice!