Sophocles` Oedipus Tyrranos is in many ways a classic murder mystery, in which the past is gradually revealed by a process of investigation. The steps of the investigation inexorably move towards revelation of the grim truth that Oedipus fulfils the prophecy and curse on his house by marrying his mother and killing his father. Dramatic suspense is provided by Oedipus constantly misinterpreting evidence. In the passage you cite at lines 1280-1290, Jocasta rushes off to commit suicide having realized Oedipus true identity. Oedipus, however, thinks she is merely concerned because she is worried that he is not noble but perhaps of poor or shameful birth.