Am I pregnant?

Carly
I am now 13 days late. I have take 4 pregnancy tests.. All negative.
No symptoms of pregnancy. Could I be pregnant or is my mind playing tricks on my body? 
I've been super stressed lately, so I'm not sure if that's the reason for my late period or if I'm actually pregnant! Help! 
Has anyone experienced this before?? 
Suggestions on what to do?