AF ended 8/22. Of AUGUST . BFN 8/23 & 8/28

Af was due in August 17th-22nd barely bled though for my 6days - so for peace of mind I took a pregnancy test 8/23 BFN
Then I spotted for a few minutes. I wiped after my FMU on 8/26 & it was damp, & light pink. 
Today, AUGUST 28th.. I took same brand test and got another BFN - I KNOW it's too EARLY to test. But I'm becoming a Pee on a stick Addict! Lol. 
With that being said 8/28 test from today had a white line show up within 5 minutes.
The test I took on 8/23 didn't even have any other line show up but the BFN.. 
Why does the white line show up though???