my period is 2 weeks late, brown discharge, no stress, still no period.

My period is 2 weeks late. I just turned 19. Around the time I was supposed to get my period I had sex with my boyfriend. But I also had exams during the past 2 weeks, which was stressful. But I am done my exams, and I still haven't gotten it. One night I had terrible stomach pains and I threw up (which sometimes happens before I get my period). The next day I started having brown discharge, but three days later I still don't have my period.
Why haven't I gotten my period?