BFP then only control line?

It was a BFP on a wondfo. I saw the line; my husband saw the line. It was more defined on the bottom and faded more toward the top. A few minutes later it was barely there and then it was gone a few more minutes after that! And I am on my period but it is light and and seems over after a couple days. I decided to test because i was getting sick from smells today. I'm confused that the line would show up because it needs to detect the hormone, but then why fade? Does anyone have any experience with this?