What Causes Delay In Periods, Headache And Nausea?

My period was due around the 6th of June and still has not come. My last normal menstrual period was May 9, 2010. I've been having headaches and nausea and a few weeks ago I remember breast soreness and I even had diarrhea (though I'm not sure that's a sign of pregnancy). How can I tell if I'm pregnant if the tests I've taken so far have all come back negative?