I think Fleur knew what the girls at the Burrow thought of her. I think she noticed everything they did when in her presence; the grunts and the rolling their eyes and the mimicing. I think she overheard everything they said behind her back, like when Hermione criticized her effects on men, or when Ginny called her a cow, or when Mrs Weasley said she wasn’t good enough for Bill. I think all this made her cry a lot when she found herself alone at the Burrow.