spotting before AF

I've been having super light spotting for about 4 days now. It's gone from gooey and brown to a little lighter at times. It won't show up on a pad. Only occasionally while wiping or when using a finger to check cervica mucus. AF is due in 3 days so this start about a week before my period was set to start. Any ideas on what it could be ? My periods are always heavy and red blood.