I used first response with this bub and at 5 days past a 3 day embryo TF (8dpo) i got a super faint line it was the faintest line i have ever seen but if there is something there and you can see the line it would only be there if it was detecting HCG.
My tests got darker each day and i have found with each healthy pregnancy i have had a line verrrrry early good luck i think you have a BFP as i can see faint lines on your HPTs