when Frank said after the Eltons walked off --

'"Happy couple!" said Frank Churchill, as soon as they were out of hearing: "How well they suit one another! Very lucky -- marrying as they did, upon an acquaintance formed only in a public place! They only knew each other, I think, a few weeks in Bath! Peculiarly lucky! for as to any real knowledge of a person's disposition that Bath, or any public place, can give -- it is all nothing; there can be no knowledge. It is only by seeing women in their own homes, among their own set, just as they always are, that you can form any just judgment. Short of that, it is all guess and luck -- and will generally be ill-luck. How many a man has committed himself on a short acquaintance, and rued it all the rest of his life!"'

Jane must have thought that unlike Mr Elton, who was happy with the wife he had known for aonly a few weeks in a public place, Bath, Frank regretted the fiance that he had also known for a few weeks in a public place, Weymouth. That part of his speech -- "only by meeting women in their own homes, among their own set" is very hurtful to Jane, it is like a pointed snub to her grandmother's home and set. Jane must have thought that the last sentence means that Frank, having committed himself to her on a short acquaintacne, is ruing it. No wonder she responded as she did -- she was the opposite of Lucy Steele, who cared only for the Ferrars money and would hold Edward on to the engagement -- Jane was an honourable young lady who would release the man from the engagement if he regretted it.