What Causes Reddish Brown Spotting While HPT Is Negative?

Hi. I am a married overall healthy 27 year old woman with a 28 day cycle. This past month I was due April 7 but didn t get my period as normal. Usually I tend to have light bleeding about 2-3 days before that increases heavily and then my regular bleeding for 7 days. This month nothing happened until the 10th when I noticed pinkish/reddish brown spotting only when I wipe. At first I thought my normal period was coming but here I am on the 13th with the same spotting and no increase in my flow. I took a hpt on the 9th and it was negative. What is wrong with my cycle? I feel crampy on and off at times but nothing..