What Causes Delay In Menstruation Other Than Pregnancy?

Hi, I was wondering i haven't had my period since the 25th of sept. I took 2 pregancy test and they both said no. I took them a week and a half apart. I was just wondering if it could be caused by stress or is it because I stopped talking my pills in the summer. I am not sexually active, I only had one partner one time since I stopped taking the pills.