Answer: Elizabeth is in agony because she doesn´t want to admit she already knows about Proctor´s relationship with Abigail.
Step-by-step explanation:
Elizabeth is being questioned by Danforth about whether or not she suspected that Proctor (her husband) had been cheating on her with Abigail. She doesn´t want to say that to prevent him being punished for it. Little does she know that Proctor already confessed, and her denying the truth puts him at risk.