Help!

Ladies I need your advice

I am as regular a clockwork and it normally lasts 5 days but this month I was due my period on Sunday. However, all I had was very light spotting (mainly brown discharge sorry if it’s TMI) but not enough to use any sanitary products and it stopped yesterday. This is not what I would normally expect. I thought it may be an implantation bleed, however, I did take a test but nothing seems to be showing up. I’m a little confused as this doesn’t normally happen. Has this happened to anyone else just so I can put my mind at rest?