The last chapter of the book is a short selection of "Excerpts from the Official History of Consuelo Camacho Ramos" (20.1). It essentially provides a brief, alternate history of Connie—the way the doctors and officials would write her life, rather than the way she'd write it herself.
Connie thinks she's sane and that she receives visitations from the future (or from several different futures) and that she hit her niece's pimp because her niece's pimp is an evil, awful person, and that she killed a bunch of doctors because they were trying to screw with her brain. The official report, says, nuh-uh, no way. You're not entirely right in your mind, Connie. You are violent and confused. Let us fill you with major tranquilizers and make you all better, OK?
So who's right, Connie or the doctors? You don't know for sure… but in the last paragraph, you learn that maybe it matters less which version is true, and matters more who's got the power. "There were one hundred thirteen more pages" of doctors reports, the last paragraph says. "They all followed Connie back to Rockover," where she'll be kept in confinement in the asylum.
Connie's own story (the novel) is over, but the doctor's reports follow her. The doctors have the power; they ultimately get to tell Connie's story and decide who she is. Her story may be that she saw the future, but their story is that she's schizophrenic, and their story wins because they're white and male and have money and education, and she's a poor Latina.
And that, just in case you're wondering, is what is technically defined as a "bummer ending."