In Act III, Scene 5, Juliet learns that her father has arranged for her to be married to Count Paris. She becomes distraught and refuses to obey her father's wishes because she has already secretly married Romeo and even consummated the marriage the night before. She tells her mother, "Now, by Saint Peter’s Church, and Peter too, / He shall not make me there a joyful bride!" After Lord Capulet learns of this refusal, he berates his daughter and threatens to disown her and throw her out of the house if she does not acquiesce. Her mother also dismisses Juliet's pleas that they postpone the marriage and so Juliet turns to the Nurse for advice. Even though the Nurse knows all about Romeo and how much Juliet loves him, she urges the girl to forget him and marry Paris. She says, "I think you are happy in this second match, / For it excels your first." Juliet is immediately taken aback by the Nurse's comments, and after the Nurse leaves, she curses her, informing the audience that she and the Nurse are no longer close:

Ancient damnation, O most wicked fiend!
Is it more sin to wish me thus forsworn
Or to dispraise my lord with that same tongue
Which she hath praised him with above compare
So many thousand times? Go, counselor.
Thou and my bosom henceforth shall be twain.

Juliet is overwrought and frustrated.  She has trusted the nurse with her secret love of Romeo and even her secret marriage. But, now her trusted companion is encouraging her to commit a grave sin by marrying Paris even though she is married to Romeo.

She feels that the nurse is two-faced and untrustworthy.  So, she leaves to go to Friar Lawrence to make "confession" for making her parents so mad.  But, Juliet is likely to do something rash simply because she is so upset, has not slept, and has lost the trust in her most trusted advisor.

Juliet simply cannot believe that the nurse would agree to her marrying Paris. I think Juliet expected the nurse to tell her parents that she was already married, but then, that would put the nurse's position with the family in jeopardy since she was supposed to be watching out for Juliet.