I have sometimes wondered if Mr Collins, in order to avoid Lady C.'s censure, greatly exaggerated Lizzy's condition which then prompted Darcy to hurry to see what could be done in the way of fetching the apothecary, only to be surprised at being let in to see her.

It would account for his hurried manner of enquiring after her followed by his momentary loss of anything else to say. His feelings, the one which could not be repressed, could easily have been agitated to the boiling point by the imaginations of a lover over a loved one being ill.