My Period Is Late - Help!!

My period is four days late! It was supposed to start the 17th... But it still hasn't come! A week before it was supposed to start I went in to the doctors for birth control and they told me I had to wait until my next period because I was going on a trip to Oregon right when this one was about to start.. The pregnancy test came up negative. Could this be due to me traveling from Arizona to Oregon with the altitude changes? Also, I've been very stressed out. My boyfriend and I had unprotected sex twice after that, help!!! Is there a possibility I could be pregnant?