The narrator is suffering from some kind of mental illness, though the field of mental health during the era in which the story takes place was woefully underdeveloped, especially when it came to women's health. At the time, women were often infantilized, especially in regard to their mental and emotional concerns. The narrator says early on that her physician husband "does not believe [she is] sick!" He has told her friends and family that "there is really nothing the matter" with her except that she has a "temporary nervous depression—a slight hysterical tendency." Her brother, she says, is also a well-respected doctor, and he says the same thing. They seem to think that she is in control of her health, that she can simply choose to be "sick" or not. At one point, John even says to her, "Bless her little heart! … She shall be as sick as she pleases!" Such expressions belittle and demean her as well as undermine her judgment as an adult who is experiencing real symptoms of mental illness.
In all likelihood, the narrator is dealing with what we would now call postpartum depression, a condition that affects a significant number of women who have recently given birth to a baby. We know that the narrator has just become a mother, as she references the "dear baby" who is cared for by someone called Mary, presumably a nanny or wet-nurse. Of the child, the narrator says, "I cannot be with him, it makes me so nervous." Subjected to what seems to be the "rest cure," pioneered by doctors such as Weir Mitchell (who is referenced in the story), the narrator's condition grows dramatically worse until she experiences a complete mental breakdown.