A common problem is that we can grip the violin neck too tightly with the thumb. This is because the thumb acts as a counterbalance when the fingers tap the fingerboard.
Play with the thumb underneath the neck, touching with the nail joint, so that you are aware of how much pressure you are using. Feel the difference in thumb contact between playing with articulated fingers and lightly shifting between positions.
The outside of the thumb is more bone than flesh which provides an incentive not to press too hard!