Had the same problem. Actually I still have it when I go heavy weight with the bar. What happens is when u grab the bar..ur wrist/hand cant move the way it wants to go because of the bar..and the wieght puts a lot of pressure on it... Im not sure if Im right. Thats what I think at least..because when I replaced bar with dumbbells ...I had no pain..at all..even with really heavy weights. I bet u someone will answer for that question.
Take care