What is suspicious about Dimmesdale’s position in his debate with Chillingworth about sin?

Chillingworth engages the minister in a conversation about why men keep secrets in their hearts rather than revealing them immediately. Dimmesdale clutches his breast and struggles to avoid directly answering the questions Chillingworth poses. Dimmesdale's discomfort is like a beacon for Chillingworth's suspicion. The minister couldn't be much more obvious in his guilt.