This may sound strange, but whenever I get it in my head that I might be pregnant (even if it is completely unlikely because we used protection or whatever), my period will be late. It has happened for a week or even two. It has happened to me at times in my life when I wasn't trying to get pregnant and also when we were trying to get pregnant (which took us a long time, so I was always hopeful - and if anything seemed like a possible indicator that our trying had worked, i'd get it in my head that maybe i was actually pregnant... and sure enough period would be late even though i wasn't pregnant). When I finally did get pregnant, the home test result was positive on about the day my period would've started.