I agree with Cheryl : check your form. You may be using heavier weights than you can really do with good form, and cheating a bit by using other muscles. One check is to compare your chest press weight to your flye weight. Flyes are isolation moves, and should use a lighter weight.
Also make sure you aren't hunching your shoulders when doing the move (something that using a too-heavy weight might encourage). Consciously keep your shoulder blades back and down and try to relax your neck and traps.
Another thing to check is if you need some kind of cushion under your head, as you might be slightly lifting your head because the surface is uncomfortable (?).
One way to help teach yourself the right form is to do the move with no weight and go through the move with tension in your muscles. You should feel the move in the chest, not in the traps or neck.