Titania means that Oberon is making up lies about her because he is jealous and afraid she is cheating on him.
Oberon and Titania both accuse each other having affairs. They are fighting because Titania has been keeping a changeling, a human baby from India. Oberon wants her to give it up. They are at a stalemate.
How canst thou thus, for shame, Titania,(75)
Glance at my credit with Hippolyta,
Knowing I know thy love to Theseus? (Act 2, Scene 1)
Oberon accuses Titania of having an affair with Theseus, and she accuses him of having an affair with Hippolyta. They each have other accusations, but Titania is basically suggesting that they are making things up about one another to irritate one another.
One of the themes explored in the play is jealousy and other negative consequences of love. Love, and obsession, can result in conflict. Titania accuses Oberon for the same reasons he is accusing her. They are angry and just throwing accusations at each other to irritate and blame the other person.
As a result of their fight, the forest is suffering. This is why Oberon decides to teach Titania a lesson and make her fall in love with a beast. It works.