Can Stress Cause Delay In Period?

Hi, I am 13 days late on my period and it will be 2 weeks late on thursday...I had my last period on Jan 6, 2011 and around the 18 or 19 i was fooling around with a guy we almost did it but did not have sex all the way because i decided not to and he was wearing a condom. I am paranoid i might be pregnant. Sometime I think I have all the pregnancy symptoms or maybe it is just me for stressing to much. I also am under pressure with school and I did have a family member die in January...so i do not know if that might be delaying my period. I have also taken 4 pregnancy test and all have been negative. What should i do?