ok last week my teacher said the way I play bass is wrong and it lead me bad habit and effect clean playing and lead me can't do muting . then he learned me how to play bass . I have smaller finger than him . he said I should put my thumb behind the neck instead of over it and grab it like bass ball bat. he said because I have smaller finger for have better reach point I should put thumb a little lower than middle of the neck . I did this and lead I can play better and move my finger more near strings but I have a problem now . when I put thumb behind the neck lower than middle then of it .my thumb that push behind the neck is ok but after few nots I start feel pain in thumb and lower of my Thumb I feel pain and it's like a muscles is in a position that it's not powerful for there or not made to have pressure there . the pain go between my thumb and ring finger have pain and discomfort. what is your idea ? Am I using wrong technique?or my muscle is weak?