My period is late...I don't think i'm pregnant though!?

So I got my period the 25th of last month.. This month I was not sexual active as opposed to other months. I've always had protected sex but now here I am 4 days late. My schedule has recently changed though. I used to work 5 to 2. Now, I wake up around noon and work at night until around 1. Does this have anything to do with it?