Answer:
Because he may not be of noble birth.
Step-by-step explanation:
At the end of the Third Episode, the messenger informs Oedipus that he was adopted by Polybus after being found as a baby. This would indicate that Oedipus might not be of noble birth. As Jocasta becomes upset, Oedipus thinks that she is upset because of his lowly birth. However, she is most likely upset because she has now realized the true identity of Oedipus.