Why is my cat so mean....?

Ok, I found my cat in Hurricane Frances, it was lost. I took care of it, never did anything wrong and one day he all of a sudden got really mean. Now 3 years later ita a mean little jerk. I never yell at him, I do everything right for him. I show him love and play with him. One second he's sweet then pow!! He nails me. His pupils get big and it gets dark around his eyes, then he meows in a very mean way!! I dont get it, he was nice then all of a sudden got mean. and to make it worse whens hes in a bad mood he goes right where im about to go then he nails me again thinking im gonna bother him but im not. I show all but anger to my cat. When Im sad and sick he nails me. When im happy and loving he nails me. He get doesn't hurt me in a playful way, he just,...just. Oh I dont know what to do!!!! HELP ME!!