Lately when I have been playing I notice that my thumb starts to hurt when I take it away from the neck and bend it if I'm done playing or something. It doesen't hurt as bad when actually playing but there is still discomfort. Also when I'm playing jazz I start to get a blister on the skinny middle pad of my thumb. I'm not sure if before I was using more of the pad when playing or if lately I have been putting too much pressure on it. Should my pressure come totally from fingers? How much of a role does the thumb play in string pressure and what part should make contact with the neck?