Implantation or not

I had unprotected sex 11 days before my period was due and he did ejaculate inside of me. A day before my period was due I had very light spotting it was only a couple drops then turned very light pink then to brown. It all lasted about 24 hours then disappeared. It's been a week and nothing.. I've had really bad heartburn lately and I never used to. Also cramps have been on and off. If I'm not pregnant does anyone know why my period would've only lasted a day? Never has happened to me before.. Always 4 days..