Will I be sinful for asking my relative to repeat what she said, as I thought she was mocking my short dress, then I changed my mind and thought she might have been amazed, not mocking?

This is a whisper [from Satan/waswasa], and you must turn away from it and not pay attention to it. Strive to gain a deep understanding of the religion, occupy your time with what benefits you, keep company with the righteous, and be careful not to sit with women without a need. If you believe that there is mockery of your garment, then there is no sin upon you if the indication points to that.