Im Using Ovia Fetility To Keep Track I Started My

im using ovia fetility to keep track i started my period last month on the 13th i believe ended the 17th 18th of july. now its telling me i shouldve started already and i usually spot then start next day and all i did was spot 2 days ago and nothing. ive taken two home pregnancy tests and both negitive. i used a due date calculator which says i would be about 3 wks