What do we know about the narrator’s past experiences that may be a potential cause for her “nervous condition?” Is she a reliable narrator?

The narrator is a classic example of an unreliable narrator.  We only 'see' the events through her writing.  Some potential causes for her breakdown include her recent birth, the fact that she is not allowed to see the child, her husband's prescribed treatment of absolute rest and mental inactivity, her sister's adoption of a domesticated female (she acts as the stereotypical wife of the time - doing housework, caring for the husband and baby), the fact that her husband discourages her creativity - refusing to let her write, and the fact that she is literally being kept prisoner in the upstairs room (the bars on the windows, the bed nailed down, her husband's refusal to move her from that room).  All of these could be viewed as reasons for her mental breakdown.