If this is the scene in Act 3 where Romeo and the Nurse talk, the answer is the Nurse feels protective of Juliet and therefore questions Romeo about his intentions toward her.
She tells Romeo that if he "deals double with her" that there will be a problem. She wants to protect Juliet, who she loves very much and who she knows to be inexperienced. It doesn't take much convincing for her to help Romeo with his plan to gather supplies so that he can sneak into her room that evening and to get Juliet to confession that afternoon to be married, though, regardless of her fears.
