It could be that you've tested to early or it could be a change in your hormones. I just went through this last month... I was ten days late and I am never late. My OBGYN said that it could be anything from IBS to lack of sleep. Your body adjusts when it's lacking in certain areas. Wait a little longer and retest. If you want to know for certain and can't wait any longer get your blood work done.
at 1:39 AM on Mar. 29, 2009