Act 3, Scene 5 follows immediately after both Tybalt's death and Romeo's banishment. When Juliet says to her mother that she is "not well," Lady Capulet immediately assumes, as does her father, that Juliet is grieving over Tybalt's death, as we see in Lady Capulet's lines:
Evermore weeping for your cousin's death?
What, wilt thou wash him from his grave with tears?
Some grief shows much love;
But much of grief shows still some want of wit. (III.v.69-74)
However, Lady Capulet further concludes that Juliet is not just weeping over Tybalt's death, but rather because "the villain lives which slaughter'd him" (81). In other words, Lady Capulet thinks Juliet wants Tybalt's death to be properly avenged and that the Capulet family did not receive the full grace of the law because Romeo was only banished by Prince Escalus and not killed as well. As a result of believing that Juliet wants revenge like herself, Lady Capulet and Juliet also discuss sending a man to Mantua to poison Romeo, which is an ironic use of foreshadowing.
Hence, Lady Capulet thinks that Juliet is grieving over Tybalt and that her grief is becoming excessive. She further believes that her grief is due in part to the fact that Romeo who killed Tybalt is not dead as well.