What can you infer from the way that Mrs. Harper reacts to Miss Strangeworth’s comment in "The Possibility of Evil"?

Shirley Jackson's "The Possibility of Evil" is a wonderful little short story that, like other works by Jackson, focuses on what happens when the idiosyncrasies of small town life go terribly wrong. Miss Strangeworth is a local gossip, but rather than share stories with others, she creates falsehoods and mails anonymous letters to those they concern, thus setting many in the town on edge and causing them to second-guess both themselves and those around them. When Miss Strangeworth goes to the grocery store, she runs into Mrs. Harper, who seems somewhat nervous. Jackson writes:

Miss Strangeworth moved slightly to make room for Mrs. Harper at the counter.
“Morning, Adela,” Mrs. Harper said, and Miss Strangeworth said, “Good morning, Martha.” . . .
“Ran out of sugar for my cake frosting,” Mrs. Harper explained. Her hand shook slightly as she opened her pocketbook.

At this moment in the story, there is no explanation for Mrs. Harper's reaction, and so it's difficult to infer what is happening. However, toward the latter part of the story, Jackson allows the reader to witness Miss. Strangeworth's writing process. In doing so, she also presents more information regarding the potential reason for Mrs. Harper's nervousness:

After thinking for a minute, she decided that she would like to write another letter, perhaps to go to Mrs. Harper, to follow up the ones she had already mailed. She selected a green sheet this time and wrote quickly: Have you found out yet what they were all laughing about after you left the bridge club on Thursday? Or is the wife really the last one to know?

Clearly, Mrs. Harper has been a target of Miss. Strangeworth for a long time. The letter she is writing in this instance is a "follow up" to many that she has already written to Mrs. Harper. The subject matter of the letter itself makes it clear why Mrs. Harper reacted somewhat curtly and nervously: Mrs. Harper has been receiving anonymous letters suggesting that not only is her husband cheating on her, but everybody in town knows it and talks about it behind her back.
In case there were any doubt as to Miss. Strangeworth's horrid nature, Jackson goes on to state that "Miss Strangeworth never concerned herself with facts; her letters all dealt with the more negotiable stuff of suspicion." Clearly, she is terrible. Fortunately, by the end of the story, she slips up and is discovered. Unfortunately, though, the reader is never granted access to the fallout, beyond the suggestion that her roses have been destroyed.