Thinking about it more, maybe it's just that Sun is hormonal (suspecting she is pregnant), and it is the writer's way of addressing the question that viewers would be asking - Why is Claire going off when she, as a mother, would stay with Aaron? The Claire character cannot do much in the show as long as she is tied to the baby all the time, so she needed a clear reason to leave him for awhile. They couldn't have her carry the baby with her.