Missed period, brown spotting

Can anyone help? I'm four days late and on my third and fourth day am getting some spots of dark brown but only when I wipe. I did a pregnancy test this morning and it's negative. I'm like clockwork and always come on at the same time so I've never been late. We've been trying for a year and although trying not to get my hopes up after the negative test I'm never late and don't know what this means?