Are you, as a reader, surprised at Mrs. Mallard's secret feeling of liberation? Why or why not?

The element of surprise in “The Story of an Hour” is composed of three parts. First, there is the shock of Mr. Mallard’s death. This is soon compounded by the surprise that Mrs. Mallard feels liberated by this untoward event. Kate Chopin makes it clear that the enormity of the emotion is somewhat surprising to Louise, but that she had had moments of wishing her life to be different. The extent of the reader’s surprise depends in part whether it seems that Chopin foreshadowed Louise’s reaction or, if she did not, whether she makes it seem believable although occurring suddenly. The third aspect is the ending, with the husband’s reappearance and Louise’s subsequent death.
From the outset, the reader learns that Louise had “heart trouble.” While this seems to be meant literally—as is born out by the ending—the author can also intend it to mean “love trouble” or perhaps a broken heart. When Louise hears of Brentley’s (assumed) death from her sister and a kindly friend, she has a “storm of grief,” weeping with “sudden, wild abandonment.” Although this reaction seems directly related to the bad news, as we have only just met Louise, we do not know if she is prone to such reactions; perhaps she tends to be overly emotional.
When Louise retreats to her room, although she is exhausted body and soul, and in her face there is “repression,” Chopin gives some hints of change for the better. Although she occasionally sobs, and her eyes have a “dull stare,” still Louise hears someone singing and birds twittering, and there are “patches of blue sky showing here and there through the clouds…” and again, her “gaze was fixed away off yonder on one of those patches of blue sky….” The intelligent Louise is thinking, and waiting. Here Chopin clearly signposts a major turn of events:

There was something coming to her and she was waiting for it, fearfully. What was it? She did not know; it was too subtle and elusive to name. But she felt it, creeping out of the sky, reaching toward her through the sounds, the scents, the color that filled the air.

Given that foreshadowing, it is logical that a revelation is at hand. The idea is going to “possess” her; she does not want it, she is “powerless” to push it away: the word escaped, “free, free, free!”
Louise quickly accepts this as a positive feeling, rejecting the idea that this is a “monstrous joy.” Rather, her feelings are “warm,” “relaxed,” and “exalted.” In case we still had any doubt, a few paragraphs down, she expands:

"Free! Body and soul free!" she kept whispering.
https://www.owleyes.org/text/the-story-of-an-hour/read/chopins-short-story