you may be overtraining your front delts. ive had this problem years ago. whenevr i did ANY chest movement, my front delts would tire and end up being so painful and worn out i would have to finish my set/workout way before my chest was done. i was using ridiculous volume on shoulders at the time, as i was 16 and thought this would make them grow. i redesigned my routine, and all the problems went. either way, it doesnt hurt to do rotator exercises also