As I tell everyone who asks a health question, you should see your doctor if you're concerned. I can provide a couple possibilities, but your doctor will have more and more accurate explanations.
Even though you say your period was four days before your bleeding that doesn't mean the bleeding wasn't just part of your period. Sometimes your menstrual cycle can last longer than normal, or it can cause bleeding some days afterward. If you've just started taking birth control pills it could be them that are causing your period to be a little out of the ordinary. Welcome to the world of birth control.
If the bleeding continues or you become uncomfortable then I suggest you see your doctor as soon as you can.
