You may very well have been trying too hard to keep your shoulders in a prescribe position. Alternately, you may have been pulling on the reins with your shoulders. Without further information or watching you ride, it is hard to tell.
The main thing to remember is to release any negative tension in your muscles. If one muscle is tense, it takes more effort to overcome this tension and move your body than if there was no tension to begin with. It also takes more time for your body to react. Negative tension is a bane of good riding.
Release the tension in the muscles of your upper body and let your body expand upward. You don't want your shoulders hunched forward, but you don't want them pinched backward either. While sitting upright, roll your shoulders back, down, forward, up, back, down, forward, up a few times. Then, stop in the up position and just let your shoulders fall to your sides.
Your upper arms should hang next to your body effortlessly. Neither pinch them against your sides nor hold them away from your body. Your lower arms, wrists, and hands should form a line along with the reins to your horse's mouth. Imagine that, if your lower arms were longer, you could hold the bit in your hands and dispense with the reins. Don't grip the reins tightly. Think of holding a little bird in each hand. You don't want the bird to escape, but you don't want to squeeze its guts out either.
As your horse moves, let your hands follow the motion of his head. This is done by allowing the angle between your upper and lower arms to open and close as your upper arms revolve in the ball and socket joints of your shoulders. Feel your seat bones move with your horse's back and your lower spine move every which way. Think: balance and moving with your horse.
You may have already learned all of these things. But, when focusing on something else, we often become subconsciously tense and forget to employ them.