I think that you are probably talking about what happens in Act II, Scene 1 of the play. This is the first time we see Titania and Oberon and they are arguing about the changeling boy. They both want to have him, Titania is not willing to give him up to Oberon.
In this scene, they each accuse the other of being unfaithful. Oberon starts it off by calling her a hussy. Then Titania says that he was having an affair with Hippolyta, who is about to marry Thesus. Oberon comes back and accuses Titania of being in love with Theseus.
Titania and Oberon quarrel over a changeling (orphan) boy. In Act II Sc.1 Oberon remarks that if she gives him the changeling boy he will stop quarreling with her:
But Titania refuses saying that the orphan boy was the child of her dearest friend who died giving birth to him and that she will never give him to Oberon:
However, their quarrel is prefaced with both of them hurling similar charges of unfaithfulness against one another. Oberon and Titania accuse one another of being unfaithful and committing adultery. Titania accuses Oberon of sleeping with "amorous Phillida" and Hippolyta while he in turn accuses her of sleeping with Theseus.