It could be implantation... could be spotting... could be AF starting... could be breakthrough bleeding... could be that you didn't O when you think you did. Not much help, huh? Sorry, but it's very hard to know. I feel for you - i know how much one can obsess over these things.

All you can do is POAS and see what it says, then wait a few days and try again, see what happens!

I hope you get the result you're looking for soon.