"All this stuff came up," a source close to Pattinson, 27, tells Us Weekly of the pair, who began fighting after the British actor blew off a birthday party planned in his honor by Stewart, 23. Dating for about three years, the Twilight costars bickered "about the cheating, about how selfish she always is, about how moody she always is," the source explains.
(Stewart and her Snow White and the Huntsman director Sanders, 43, were photographed making out in Hollywood last July. Although Stewart and Pattinson reconciled last fall, Liberty Ross, Sanders' wife and mother to his two kids, filed for divorce earlier this year.)
The source adds that Stewart's shocking indiscretion with Sanders was just one point of contention for the pair "It was a lot of issues that came up."