I think Elinor was disappointed in her overall reaction when Lucy informed her of the secret engagement. Elinor was obviously stunned and eventually recovered fairly well but I think she felt she did not react in a subtle enough manner that would prevent giving credit/confirmation to Lucy's suspicions. IMO Elinor feared she may have given Lucy admittance to her private feelings as regards Edward thus giving Lucy an authority over Elinor that she (Elinor) would like to avoid. If Lucy felt that she had indeed discovered Elinor's true feelings, Elinor's reapproach of the subject of the engagement and her subsequent campaign to reconstruct a level playing field for herself, all kept within propriety, have thrown Lucy off balance again. It is like a cat and mouse game, dealing with Lucy, and Elinor doesn't intend to sacrifice the knowledge of her feelings to the likes of a Lucy Steele. I see it as a form of damage control and perhaps a matter of pride on Elinor's part.