Blood in Syringe

Hey guys I noticed post injection after I pulled the needle out, there was a drop of blood in the liquid left in the hub. I aspirated before injecting but I didn’t notice any blood. What could this be? And why was it in there left over? If blood was pulled in during aspiration shouldn’t have been injected back in at the beginning of the shot? Here a pic.

Its happened to me subQ. All I can figure is that when the stopper got to the bottom and I eased up on there plunger there was a slight vacuum created and drew a little blood in.