Spotting vs period?

I am 12 days late. The last couple days I have been just barely spotting, where I can't even tell I'm bleeding at all except for when I wipe. Today, the bleeding has gotten slightly heavier, but I'm only using a liner and it has gotten me through my 3 hour class with ease. My flow is normally very heavy, so I have no idea what a light flow even looks like. Am I spotting, or could this be the beginning of my period (finally)? Where's the line between the two?