Positive HPT and now negative?

Celeste • Pregnant with baby #1 EDD April 3rd, 2016!
I posted a very faint positive hpt on here two days ago. I've taken two more pregnancy tests yesterday and today. Yesterday's was 100% negative. Today I could online see a faint line if I shined a flashlight behind the test. What could this mean that I am testing negative now? I was 5-7 days dpo with the first test, now I'm 7-9 days dpo. (I know, I know...I'm impatient)