At the end of the third episode Oedipus thinks Jocasta is upset because he may not be of noble birth.
After receiving the tragical news of Polybus death the messenger who delivered it start talking with Oedipus and revealing him his past. Oedipus was adopted by Polybus after being found in Citeron´s tree-lined glens. Then Oedipus ordered to the chore to found the farmer whou have found him when he was a baby.
Jocasta started to argue with Oedipus telling him to not look for the farmer, perhaps because inside of her mind she was connecting the dots and started to reveal the awful true. But Oedipus didn´t listen to her advises believing that she would be embarrased of being married with a non noble person.