Why does this happen?

I had sex 4 days before my period was due (today it is due in 1 day), and afterwards there was a clot and a fair bit of blood sort of browny-red in colour. I get it’s normal for that to happen if you’re near your period but why does it happen? I’ve never bled after sex before.

And for the past few days leading up to what I presume will be a period there has been some light brown discharge? Why is this and is this normal?

Thank you for all your help!