Positive digital test

I took a digital test Thursday night 9/14 it said

"pregnant". I just took another digital today

9/17 and it says not pregnant l've taken several test Friday and Saturday that also say

NOT pregnant. Does anybody know the reason for this?? Or has had this happen? I'm supposed to start my period on Tuesday

9/19. Ive had sex while ovulating on the 3rd and 6th so it's not impossible for me to be pregnant.