Could The Delayed Periods Be Due To Pregnancy Or Stress?

I'm 24.I should have started my period Sunday. I have been under stress but I usually start every 28-29 days. The first day of my last period was 09/19/10 & was 6 days long.its always 6day. I have taken 4 pregnancy tests,they have all came up negative. digital test which also said "-NO". I'm not showing pregnant. If I took the morning after pill at this point would that stop anything that may be going on. Is there any alternative steps I can take before that tests maybe becomes positive?