What Do Delayed Periods And Chills With Negative HPT Suggest?

My period is 4 days late, all tests have been negative. I am getting chills with no fever and I feel like my period should have come last week (hormonal cramps) but nothing. I realize pregnancy could still be a cause but with both my past pregnancies I tested positive a week or a few days prior to missed period and never had chills and body aches.