Hi I Missed My Cycle Was Supposed To Get It

Hi i missed my cycle was supposed to get it January 18 2016 but nothing so i checked on several cycle tracking apps and they all said i was supposed to get it 1 12 2016 i dont see how thats possible andi get it every 21 days so i say about 40 minutes ago i seen pink discharge at first i thought naw but then i checked again it was pink and milky white i did a pregnancy test a day after i missed it and it said negative