In chapter 23 Charlotte has accepted Mr. Collins proposal. Mr. Collins is returning from Huntsford and staying at Longbourn instead of Lucas Lodge. Which does not please Mrs. Bennet. The narrator says...."She {Mrs. Bennet} hated having visitors in the house while her health was so indifferent, and lovers were of all people the most disagreeable. Such were the gentle murmurs of Mrs. Bennet, and they gave way only to the greater distress of Mr. Bingley's continued absence."

So, in Miss de Bourgh's case it is possible it could be some kind of nerve/depressive state, seeing how she has to put up with Lady Catherine. :)