Not only was she emotionally and physically exhausted, but Elinor was expecting Colonel Brandon and her mother to walk in. In fact, she thought it was them when she heard someone had arrived.

Elinor also knows that the last time Willoughby and the colonel saw each other, they were aiming at each other with guns! One can only imagine how Brandon might react to find Willoughby there to see Marianne, alone in the house with the young ladies and no chaperone! Surely Elinor has this on her mind.

She also knows that her mother felt "a disappointment hardly less painful than Marianne's, and an indignation even greater than Elinor's", and she is also scheduled to arrive on the scene momentarily.