help !!

so my period started on 31st of august and ended on 4th September! it lasted 4 days for the first time in my life, because my period ends on either 5th or 6th day( just a drop of blood). my flo app says that my ovulation is supposed to be in 2 days but i just saw this dark blood on my pants. could this possibly mean that it’s the old blood that didn’t come out because i didn’t bleed for another day or two or what? keep in mind that i did have sex the day after my period ended.