In "Two Swords" (*Game of Thrones*, S04E01), Qyburn asks Cersei whether her "symptoms" had gotten better. When Jaime asks her about this, she refuses to tell him. They then engage in an argument in which Cersei tells him that he has arrived "too late". At the time, I assumed it was because she is engaged to Loras Tyrell but I wonder if that's not the only reason.