Can Stress Cause A Delay In Periods?

Hi, I had my last period on 28/4/11 took Levenolle 3 days after my last period, i.e. 6/5/11. However its been over 3 weeks now, still no periods, did 4 test and all negative. I have no other signs of pregnancy but I have been stressing a lot as I was also having my exams. Please help.