Advice needed!!

Daisy

My cycles vary between 27 - 29 days. I ovulated this cycle on day 13 which was 12 days ago and had sex every day leading up to then. Expecting AF in 2 to 3 days but for the last 3 days been spotting started off light pink now today dark brown. Only a little bit nothing to fill a tampon/pad. Never had this before. Tested today and negative. Any ideas what this could be?? Been ttc for a year with 2 early miscarriages feb and May last year.