Late period but negative test

I'm 8 days late and both the tests I have taken have been negative. I had cramps last week and my boobs hurt so I thought I was going to start but never did.. I've never been this late so I don't know what's going on. Anyone have any idea? I'm going to retest in the morning but I hate not knowing. And if it's negative again I don't know why else I would be so late?