Help! Which One is Correct?!

We’ve been TTC for 6 months, which is when I stopped birth control. When I did that my hormones were all out of whack. What used to be a regular cycle turned into periods being a few days late or a week early. I relied heavily on ovulation urine strips instead of the Glow calendar to determine when I was ovulating, and the coordinating HCG urine strips to test when I was late. The HCG test strips have always been negative. Not even close to having a faint line. This month at 4 days late (because my cycle finally normalized last month), I took a HCG test and it was positive. Today I went to the store on post and bought a First Response 6 Days Early pregnancy test and it’s completely negative. How is that even possible? Which one do I trust? I haven’t changed anything in my diet or lifestyle, and no medications at all.