Implantion bleeding???

What exactly is Implantion bleeding? How long does it last? Do you have cramps? 
​My period was suppose to be on the 2nd & just came today the 4th (in the afternoon), not sure if it's AF or Implantion bleeding..  
​What do you think? I tested myself yesterday came back negative.. Should I let this bleeding end & then test again??
​Any advice?