My memory of past chapters might be lacking but isn't this the first time Himawari-san has reacted, that strongly to Matsuri's love declarations? Def feels like she's finally starting to become conscious of her, not just as a child but as a woman for lack of a better wording~
That's probably true--her early responses to Matsuri's "I love yous" were either to ignore them or to be confused about what caused them.
But pretty much ever since the "Matsuri is special to me" emotional (whether romantic, Platonic, whatever) confession under the sakura blossoms, Miss Sunflower has been shown to be increasingly conscious of Matsuri and of her own feelings about Matsuri, who also has been depicted visually and thematically as significantly more mature than she used to be.
That's why I find the earlier poster's statement about "no progress" so utterly baffling--to me the latest few volumes have been almost nothing but (in the subtle and understated way of this series) significant progress in the Miss Sunflower/Matsuri relationship.