Here is a prime example of Antoinette's questionable insanity. As mentioned before, this passage alludes to the fact that Antoinette's eventual insanity is attributed to the negative way that she was treated by society. She is paranoid that "they (have) changed (her dress)", highlighting her learned distrust for other people. Due to this, it is only natural in her altered mentality that she would be suspicious of someone doing something that would corrupt the one last fond thing that she has. Her suspicion that "it (may not be her) dress at all" seems plausible up until the point that she questions "how (they could have gotten) the scent." It is now evident that the dress that she holds in her hand is actually her dress, because there is no other way that the exact scent could have been duplicated. Her paranoia in her questionable mental state has affected her judgement to the point where she is no longer able to perceive her surroundings logically.
This excerpt overall exemplifies Antoinette's altered mentality, but also points to the roots of this altering in a learned distrust for people.