If the bow fits you correctly as you say, you shouldnt get any arm slap unless your torquing the bow slightly. Make sure your bow arm is slightly bent on full draw. Post some pictures of your form so thoes of us who have been shooting for a few years can help you improve your form. It really sounds and looks to me as if its a form issue or the draw length is a bit too long causing you to torque the bow. Give us some pictures from the front, two sides and the back, and if possible one from above.