What Causes Delayed Period And Lower Back Pain While HPT Is Negative?

Hi..I am on 30 day cycle, supposed to be I ll be having my period on 6th march but missed it. Had contact on my fertile days. I have done home pregnancy test 2 days after my missed period And turned out negative.I have cramping, lower back pain and moderate bright red bleeding right now. Am I experiencing delayed period or is it something else? Thank you!