What Causes Delay In Periods And Light Bleeding Between Periods?

Hi, my period is normally like clockwork, its around the 20th each month, dec was normal, but then i hada random 2 day bleed in jan on the 6/7, i didnt have a period around the 20th jan either, my husband and i our trying for a baby but its our first so asking questions. Today though i started to bleed ligtly, does this mean my period date has changed? or implantation bleeding?