Late period :(

I had unprotected sex exactly a month ago and my period was supposed to come the 9th, I have been extremely stressed about my finals and other things right before I was supposed to get my period. I still haven't gotten my period and it is as of now 9 days late. I went to a pregancy clinic on the 13th and the results were negative and I just took another test today also negative. But I'm still worried. Could it just be stress that's delaying my period?