By Act 3 scene 5, the audience has seen Romeo and Juliet marry and even spend the night together both in secret. No one else in the story but the Nurse and the Friar know that. Shortly after Romeo leaves Juliet's bed, Lady Capulet comes in and we can tell she doesn't understand her daughter as the two talk about how and why she cries. Lady Capulet is sure it is because Juliet lost her cousin. Then, in an effort to soothe the pain, Lady offers Juliet the opportunity to marry Paris. This was a total set-up. Lady Capulet (on her husband's behalf) is just using the circumstances.
Any girl would want to tell her mom about the true love she found, but Juliet can't because she knows it is a forbidden love. Thus, there is division between the two. As the scene progresses Capulet gets involved and Lady Capulet turns on Juliet and her wish to not marry Paris, Lady determines to take dad's side.