Chances of pregnancy?

I had unexpected (unfortunately) unprotected sex on the 6th of this month, (he didn’t finish inside) and 16 hours later took Plan B - I have a few different period apps because my cycle length is always different every month .. some apps say my period was expected on the 16th and some say my period is expected on the 20th - for the past two days there’s a tiny minute amount of light brown discharge that almost seemed to have a tinge of blood, I took a pregnancy test this morning that resulted in negative - has this happened to anyone? Brown spotting and then your period - or delayed period after taking plan b? Thank you!