What does Lady Capulet think is the reason for her daughter's sadness? What is her suggested solution and does it help matters ?
I just want to add to the question by saying that Lord Capulet has moved the wedding date up because he feels that Juliet is distraught over Tybalt's death and by marrying Paris, sooner than planned, it will put an end to her woes.
Lady Capulet feels that Juliet is more distraught that the "villain who slaughtered Tybalt lives." Lady Capulet wants to send someone to poison Romeo and Juliet tells her mother if she finds the person to do it, she will mix the poison. There is some indication that what Juliet says could mean one thing to her mother and another to what she actually means in reference to her and Romeo. Lady Capulet then says, after she carries out her plan to get rid of Romeo, she has the real solution to make her happy and that is that Lord Capulet her father has arranged for her to marry Paris. Juliet then gets into a verbal confrontation with her father and he threatens to sever all ties with her to the point she could be homeless, starving, ill, etc. and he still will not acknowledge her. Her mother also tells her that she has washed her hands of her and if she doesn't marry Paris she has no more family. Even when the nurse tried to stick up for Juliet she was told to go tell her gossip somewhere else because Lord Capulet said she is not needed even though she is the one that raised Juliet.
Source: the Language and Literature Book by McDougal Littell
Lady Capulet believes Juliet's tears and puffy eyes are over the death of Tybalt. She then tells her she is sending someone to poison Romeo, and that her father has agreed to let Paris marry her the next Thursday.
Juliet is very upset with this news, and ends up in a violent exchange with her parents.