Bleeding after sex?

hellopris

So I’ve had my period 10 days ago & I’m currently on my fertile window. I had sex 4 days after my period ended then I started noticing brown-ish leftover period blood. This continued for 4 days, today’s the 5th day & it’s full on red like my period is back! Any of you know why does this happen? Is this normal? I’m freaking out!!! :(