Lady Capulet has no idea what has been happening in her daughter's life (or heart).
The last conversation Lady Capulet had with Juliet prior to the party was about marriage (to Paris). At that time, Juliet had told her it was an honour she dreamed not of, meaning it was not something that she aspired to or dreamt about. Lady Capulet rightly thinks that Juliet is devastated over the cruel murder of her cousin Tybalt, which, in part, she is. What Lady Capulet doesn't know is that Juliet, who said she wasn't interested in marriage, has already met someone, fallen in love, and married (without telling either of her parents). In Lady Capulet's defense, the misunderstanding is not at all helped by Juliet, who plays word games with her mother, telling her truths while leading her to continue with her mistaken interpretation of the reasons for Juliet's sadness. Juliet says she would rather marry Romeo (her cousin's killer) than Paris, which is true but not at the level her mother takes it. She also says she wants to bear the poison to Romeo herself, again leading her mother to think that her hatred for Romeo is intense; we know she just wants to see him and, to intervene against her family's attempts to murder him in revenge.
Finally, Lady Capulet herself seems quite enamoured with Juliet's suitor, Paris (she says Verona's summer has not seen so fair a flower as him); it is also possible, therefore, that she simply cannot conceive of Juliet falling for anyone else. In her mind, there could be no other logical reason for Juliet's state of mind than the immediate one before them.