In additon to what Janet R wrote, I add this: I understood sentence on first read to mean Mrs. Bennet thought that for sure something had happened to Lizzy, that she was lying in a ditch somewhere.
If you read it to mean Mrs. Bennet had placed Lizzy in a ditch, the context should make it apparent that this is an incorrect surmise. It does not need to be rewritten, in my opinion.
