AF, Ovulation Spotting, or Implantation Bleeding?

Today is CD15 for me. I started spotting yesterday morning - light to medium spotting, going back and forth between pink and brown. Unless I ovulated very early, I doubt it's implantation. My concern is that it seems to be getting heavier. I only see it when I wipe; there's nothing on my panty liner. I'm also having mild cramping. So is this possibly the start of AF coming two weeks early, or do you think I'm ovulating? We've been BD'ing daily just in case!!