The Sun Also Rises: Chapter XVI~Character Relationships

What is wrong with Brett? She seems to have a close relationship with every other character in the novel. First, she and Jacob kind of had a thing, but for some reason that didn't work out. Next, she moved to Cohn, taking a vacation with him to San Sebastian; although, later she hates Cohn. Finally, she has a thing for Pedro Romero, the young bullfighter (15 year age difference!). All the while, Brett is going to marry Mike. How does Mike feel about this? Is this foreshadowing that Brett will continue to move from man to man and never be able to settle down?