This is a good question because Hester encounters a lot of mixed emotions at different instances regarding Dimmesdale.
In this particular chapter, Hester has basically gotten "the message" that Dimmesdale was not going to fulfill his promise of going away with her. First, Mistress Hibbins, the town's witch, tells Hester that she knows about the meeting between Hester and Dimmesdale in the forest.
“Dost thou think I have been to the forest so many times, and have yet no skill to judge who else has been there? Yea, though no leaf of the wild garlands which they wore while they danced be left in their hair! I know thee, Hester, for I behold the token. We may all see it in the sunshine! and it glows like a red flame in the dark. Thou wearest it openly, so there need be no question about that. But this minister! Let me tell thee in thine ear!
Second, the captain of the ship that Hester had booked to escape with Dimmesdale discloses that Chillingworth booked himself in too, which means that Chillingworth also knows that Hester and Dimmesdale were trying to elope. Moreover, the people of the settlement were particularly mean to Hester especially as they saw her speak to the sailors who were spending time in the town during the festivities.
All of these events are significant because they all serve as a benchmark that can help predict what will happen next: that Dimmesdale changes his mind and completely ignores Hester as he approaches the pulpit to give his speech. He is aloof at the time of his presentation, and totally bypasses Hester.
...so abstracted was his look, it might be questioned whether Mr. Dimmesdale ever heard the music. There was his body, moving onward, and with an unaccustomed force. But where was his mind?
Meanwhile, Hester is standing on the side looking for a non-verbal sign from Dimmesdale that would verify that what they spoke about in the forest was still in place. They had spoken about eloping, about starting over, even about becoming a family. Now we realize as readers that Dimmesdale is too involved in his idealized role as a pastor to give any value to his role as a man who failed a woman.
Hester Prynne, gazing steadfastly at the clergyman, felt a dreary influence come over her, but wherefore or whence she knew not, unless that he seemed so remote from her own sphere, and utterly beyond her reach. One glance of recognition she had imagined must needs pass between them.
This being said, Hester is in shock and obviously RESENTFUL that Dimmesdale yet again demonstrates his lack of manhood and character. She is trying desperately to figure him out as the roles that society has bestowed upon him: those of clergyman, man, leader, and overall counselor to a lost herd. Yet, she is slowly (unbeknownst to her) realizing that this man is far from the idealized hero that the settlement has made him out to be.