That never occurred to me. I suppose, if Charlotte had left Elizabeth's indisposition vague it might be so. But had she said 'headache,' I do not think Darcy's panic would have been triggered. And, if he thought it were a desperate illness, he must have suspected Charlotte would not have left her to the care of servants, or hid her agitation and concern in social civilities. Indeed, Elizabeth has been the butt on Caroline Bingley's jokes as being frightfully, unfashionably independent - she certainly has never been a frail flower in the time that Darcy has known her.