She feared it was a strengthening regard.

It grieved her to see the earnestness with which he often watched Marianne, and his spirits were certainly worse than when at Barton.
About a week after their arrival, it became certain that Willoughby was also arrived.

His card was on the table when they came in from the morning's drive.
"Good God!" cried Marianne, "he has been here while we were out." Elinor, rejoiced to be assured of his being in London, now ventured to say, "Depend upon it, he will call again tomorrow." But Marianne seemed hardly to hear her, and on Mrs.Jennings's entrance, escaped with the precious card.
This event, while it raised the spirits of Elinor, restored to those of her sister all, and more than all, their former agitation.

From this moment her mind was never quiet; the expectation of seeing him every hour of the day, made her unfit for any thing.

She insisted on being left behind, the next morning, when the others went out.
Elinor's thoughts were full of what might be passing in Berkeley Street during their absence; but a moment's glance at her sister when they returned was enough to inform her, that Willoughby had paid no second visit there.