While Benedick and Don Pedro are talking, Beatrice begins to walk toward them. Benedick quickly excuses himself. He does not want to be around Beatrice, and so he quickly walks off.
Don Pedro is amused by Benedick's behavior, and he jokingly tells Beatrice that she has "lost the heart of Signior Benedick" (240-41). Don Pedro does not know that Benedick really did love Beatrice. He is just making this humorous comment because of the festive or carnival mood that dominates the night. In fact, he is using understatement to suggest that Benedick hates Beatrice.
Beatrice, though, responds as if Don Pedro had spoken the literal truth:
Indeed, my lord, he lent it me awhile; and I gave him use for it, a double heart for his single one. Marry, once before he won it of me with false dice. Therefore your grace may well say I have lost it. (242-45)
Beatrice, using a loan metaphor, is stating that she had a relationship with Benedick in the past, and she had responded by falling in love with him. The word use suggests interest on a loan. When Benedick had loaned Beatrice his heart (had a relationship with her), she repaid him by giving him both his own heart and her own (two hearts or "double heart"). She is indicating that she fell completely in love with him.
But then she adds another metaphor -- this time, a gambling metaphor. The game of craps is a form of gambling that requires two dice, and the person rolling the dice is betting that the total of the two dice will be a certain number. False dice are dice that contain a hidden weight inside so that the number will always come up the same. False dice are used to cheat. Beatrice, then, is claiming that Benedick had cheated her in the past. She is claiming that Benedick pretended to love her, but that she had truly loved him. Beatrice had gambled on love, and had lost.
Of course, what Beatrice does not know is that Benedick really did love her, and still does. The details of their break up are not revealed in the play. But given the witty and sarcastic nature of both of them, they probably caused their separation by speaking sarcastic words and by some sort of misunderstanding.