If you got a positive opk cd 11 (I think you’re now cd 18 if my maths is right?!) you could have ovulated that day possibly, and the reason your opks stayed dark was because you ovulated both sides? So if you conceived cd 11 or 12, implanted cd 16 or 17, (especially if it is 11 and 16 as that is the lower end of ‘average’ - six days) which is early but not impossible, then getting a line today isn’t implausible for a healthy pregnancy, especially if it is twins or a really healthy sticky bean. I’d be more calm with an early BFP than a late one, as I’ve always found late implantation an indicator that it’s not going to last. Be happy! Xx