Why does Mrs. Mallard feel so “Free! Body and soul free” in "The Story of an Hour"?

When Mrs. Mallard learns of her husband's accidental death
in Chopin's "The Story of an Hour," she weeps with "sudden, wild abandonment," and then
she goes to her room alone, for "[S]he would have no one follow her."  Alone upstairs in
her room, facing an open window which looks out onto the spring day, Mrs. Mallard
feels

pressed
down by a physical exhaustion that hauted her body and seemed to reach into her
soul.

In contrast to these
feelings, Mrs. Mallard feels "the delicious breath of rain" in the air and hears the
happy singing of birds.  Clearly present in these passages is a subtle conflict.  For,
while downstairs, Mrs. Mallard has abandoned herself in weeping, but then gains control
and retreats to her room.  Now, in her room she sits motionless, but a sob finds its way
into her throat, and she abandons herself again.  However, this time the sob is for
herself as she dully gazes "off yonder on one of those patches of blue sky." a symbol
of the future. The brewing storm of emotions within her, "something coming to her and
she was waiting for it, fearfully," resists any control that Mrs. Mallard can exert
because of her social mores.  She is almost giddy as the realization that her life will
no longer be dictated by the patriarchal society Mr. Mallard represents, under which her
individuality has been repressed, comes to her.  Under her breath, she whispers the
words she dare not shout in joy, "free, free,
free!" 

The contrast between the joy that Louise Mallard
feels as she utters these words and the "look of terror" that follows results from the
repression of Mrs. Mallard as opposed to the newly realized freedom of Louis Mallard. 
Knowing that she is no longer subjugated elates her, but she also knows that she must
act properly for a widow, and she does love her husband.