Day 6 still haven't started!

Today is the 6th day after I was supposed to start my period. I'm extremely regular, as in I start the 19th every month! I've been really active, lots of streas(but it's been the same for the past three months.) I've taken 5 at home pregnancy tests to see if I'm pregnant, but they all came up negative. I took one the day before my period then one every other day. Took one again just now and still negative. :( how soon should I speak with my OB about this? I know stress could be a big factor, but like I said, it's not more than the usual. Is it possible to still be too soon to test positive?