Who told lady Catherine. I am sure you have already read this quote from Chapter 57.

"It was a rational scheme, to be sure! but from what the report of their engagement could originate, Elizabeth was at a loss to imagine; till she recollected that his being the intimate friend of Bingley, and her being the sister of Jane, was enough, at a time when the expectation of one wedding made everybody eager for another, to supply the idea. She had not herself forgotten to feel that the marriage of her sister must bring them more frequently together. And her neighbours at Lucas Lodge, therefore (for through their communication with the Collinses the report, she concluded, had reached Lady Catherine), had only set that down as almost certain and immediate, which she had looked forward to as possible, at some future time."