OPK QUERY

Corrina • have 4 kids with partnerthat passed away 7 yrs ago. me and my OH are TTC our first baby together

so my period finished yesterday but this afternoon I got a positive on my opks

Can anyone tell me why?

I know u can get positive day your af is due sometimes but my opks are usually blank after af up to ovulation and day 13-15

Any answers would help