What could it be?

Yesterday I started what I thought was my period. So i put a tampon in. I had to sleep with one in. It never got heavy. It was brown. I had unprotected sex a week ago yesterday. Now the bleeding has stopped. What was it? My period or implantation? I've also had on & off very sharp pains in my right breast. Idk if that's a sign for anything but I thought I would throw that out there