What caused Joetta to leave the church early?

In chapter 14 of The Watsons Go To Birmingham, being too hot at church protects Joetta from the bomb. Since the novel is told to us from Kenny's point of view, he tells us about that Sunday and its events. He tells the reader that he knows it was a Sunday, because he saw Joey getting ready for church and remembers her dress, lacy socks, and shiny black shoes. That morning he is taking a nap in the backyard when the noise wakes him. Kenny hears from Byron that someone dropped a bomb on the church, so he runs to the church to see what happened. Once he arrives there, he sees the chaos and people everywhere. In horror, he sees a shiny black shoe just like the one she was wearing that morning. He believes the worst: that she is a victim of the bomb.
Kenny goes home and tries to remember how he treated her that morning. He doesn't believe it when he looks up and sees that Joey is alive. Once he realizes that she is alive, he explains what happened, and she explains that she had to leave the church early because she was too hot. She tells Kenny, “It was so hot in there that I went and stood on the porch and saw you."