Usually When I Have My Period I Bleed Heavily, But

Usually when I have my period I bleed heavily, but yesterday I started bleeding very very lightly. It was also odd because it was a few days before I was supposed to start, and I've never experienced spotting in between periods before. I've tried to do some research on this.. Some websites say this could be implantation bleeding and others says it's not. I'm really confused about what this could mean.