Could be nearly anything. Bow hand grip, bow arm straight but not locked, bow arm elbow pronated, bow shoulder down and back, etc.
A heavier bow COULD cause you to change from what you are doing from light to heavier draw weight. Also, if you pulled a muscle in your neck, your shot may need some work. The neck muscles should be largely out of the picture with a good draw and shot execution.
We really need to see a few shots on video to help much. One or two of your shot with the lighter bow and one or two with the heavier (if you dare). Wear an arm guard!
Arne