Hyperextending means that the elbow joint is bent beyond 180*, in other words, bent a little bit past straight in the wrong direction. This is fairly common in women and girls. Although I have yet to see it in a man, I suppose it is possible, and yes, it can cause string slap. The best cure is to use an arm guard, as it is difficult to hold the arm at exactly 180* if it wants to bend farther.
More likely is that you are rotating the elbow joint such that the inside of the elbow is facing up. The inside of the elbow should be facing more or less towards you, with the point of the elbow facing away from you. If you have this problem, try consciously holding your elbow in the correct position until it becomes natural for you.