I feel like it was in character unlike what people are saying here though... A life as horrendous as Leslie's, the smallest grace in her darkest times, of course, she will remember the person and their deed and will unconsciously think they are truly kind. You guys have seen it in so many novels and comics, where the saving grace of someone at their darkest time, no matter how small, will be deeply imprinted in them.
Of course she feels mad that they were doubting her benefactor in that manner, it would be weird if she wasn't, if she were a cold and ruthless character it may have not been the case, but she's kind hearted. She feels sad and insulted that her benefactor was doubted like that, and I think it would have been out of character if she had let the knights "humiliate" her in that manner.
A very simple comparison would be, people testing the food that your parent (mom or dad) for poison, I'll tell you, if someone did that to me with the food that my parents gave me, I'd be angry, so I think Leslie's in a similar state.