I assume that you are right handed. You are not used to using your left hand. Take it slow and give it time. Don't try and force your left hand, It will only make your wrist sore. Hold the stick loose in your left hand and act as if you are playing without hitting anything. Move your wrist up and down slowly as far as it will comfortably go. Pinch the stick at the bottom of every stroke with your thumb and forefinger if you are playing matched grip style. This will strengthen and stretch the muscles in your wrist. Again, don't force your wrist. Take it slow.