Late period AND negative test

My period is like clockwork, arriving every 26-27 days. But now I'm 3 days late. 3 days may not sound like much, but it's a lot when I haven't been 3 days late in years. I took a pregnancy test and it was negative. Heartbreaking. We've been trying for 4 cycles. Negative AND a late period?? It doesn't make sense. Has this happened to anyone else?  Does anyone know why it happens?