Very confused.. period and BFP?

Not sure what is happening here.

I tested last month, ovulation happened Cd17 but BFN a couple days before AF arrived.

Got AF as scheduled March 1-4. Because we are trying, I started taking opk on March 6. Positive. Positive for the next 4 days, so I thought why not take a pregnancy test for giggles. BFP!

Wth. Doesn’t even make sense. How can I have a positive test 4 days after I finish my period? (And no we don’t Bd while on my period)