got my period two weeks early? day after I had sex?

I'm confused as to why I got my period two weeks early. I haven't had sex in about a month and the day after I did, my period started. Two weeks early. 
My periods have been regular for as long as I can remember. I haven't been on birth control since February. 
I read about implantation bleeding. But that usually comes 6-12 days after conception. I took a p test and it was negative. 
This is the same partner I've been having sex with. Unprotected. Withdrawal only. 
Does anyone have any ideas?