What is happening to my body??

So I had unprotected sex on the 3rd of October but protected sex every other time

I am pretty on schedule with AF starting around the 18-21st of every month.

Well today I had that brown stuff that I always get before AF , well I had sex tonight and started bleeding a lot as if AF has arrived.

I have also been taking opk and have been negative. I started testing on the 5th because that was the predicted day on here but has been negative since then.

What is happening ?? I haven’t or don’t have any AF symptoms.