It may not be fair that you are getting charged for what was essentially you trying to protect yourself.  However, if you are claiming self-defense then it will be your burden to prove that you did not act unlawfully. California Penal Code section 693 essentially gives a person the right to use reasonable force to repel an assault.Here, assuming that you came up to the bully peacefully and were not the initial aggressor, the bully arguably committed assault upon you when he got into a fighting stance.  Once he made you fear that he was going to hurt you physically, you had a right of self-defense to use reasonable force to repel that assault.  Now, reasonable force is an objective standard based on the circumstances, and the tricky part of your case is that you hit him again when the bully was already on the ground.  However, a crafty attorney may be able to portray you as a reasonable person and say that your fear that he was going to get up and hurt you was reasonable under the circumstances.