Jill's inconsistent affection

Can someone explain to me what changed for Jill? At the start she seemed overly flirty in the shop to try to get him on the team. Then she's quite friendly and clearly interested, but rejects his kiss on the train. It's made quite clear that she's not interested (despite flirting up a storm). Then suddenly they're in a club and she's all over him, but nothing seems to have happened to prompt this change. What gives?