Desdemona cannot believe Othello's accusations. She has no clue why Othello would possibly be saying such vile things to her and why he would call her a "whore" as he does in this scene. She is shocked, hurt, and incredulous:
When Desdemona enters, and Othello accuses her of being a whore, she is taken aback and wonders why he is acting so belligerently. When she admits only innocence he says “I took you for that cunning whore of Venice / That married with Othello.” This interaction between the two of them is a direct contrast to a previous scene in which they were both overjoyed at seeing each other on Cyprus. The conversation underscores the drastic change Othello has undergone with Iago’s insidious plan, and it emphasizes the deterioration of his belief in her fidelity. (Enotes)
In this same scene, Desdemona meets with Iago in order to get possible answers as to why Othello might be acting the way he is. Iago slyly blames it on the "business of the state." Regardless, Desdemona is heartbroken and is desperate to win back Othello's trust and prove her innocence.