Mrs. Brown has "spells" because she has not been able to fully recover from the trauma of her only child's death two years ago. In Elijah's words, the baby, a two-year-old boy, had "died hard of the fever...and ever since that happened, Mrs. Brown (is) being bothered by spells".
Although most times, "you wouldn't've knowed there was nothing plaguing her", Mrs. Brown really does suffer from the lingering psychological effects of her loss. She wears black all the time, and once told Elijah's mother that she wasn't going to start dressing in colors again until the Lord "blessed her with another child". Sadly, Mrs. Brown has been told by multiple doctors that she will never bear children again, but she apparently has not been able to accept that fact. Mrs. Brown does other things that show evidence of her still fragile state of mind as well. Sometimes, in the dead of night, she will go out into the woods and just stand "leaning up 'gainst a tree, humming and rocking to and fro with her arms wrapped 'round herself", or she will squat down in the dirt and brush at a spot "that didn't look no different from any other spot in the woods...till it waren't nothing but hard earth".
Most of the time, however, Mrs. Brown acts perfectly normally. She is kind to everyone, and, as Elijah notes, "caint no one in the Settlement bake the way she does" (Chapter 5).