Got any pics? Do a check in the mirror, it may be a case of one trap sitting literally higher than the other one. Check your shoulder height facing the mirror facewards...You may notice that one shoulder sits higher than the other one and that one side is just "raised" if you like in general. You may have scoliosis. Other than that, i suggest that it's just simply a muscle imbalance between your left and right side. the upper portion of your left trap may be significantly weaker, the lower part stronger than the the lower part on the right side...or both.
