Amy was being WAY too obvious for Karma not to realize (which it seems she did at the end). I don't know how she wouldn't notice Amy not taking her eyes off her when they were lingerie shopping. Speaking of which, how cute was Amy during rehearsal when Karma took off her coat to reveal her standing there in a wife-beater & flannel shorts?
I think that Karma's reaction at the end had way more to do with Amy than with Liam. After the kiss, Karma looked almost elated & like she wanted to go in for more...when Liam jumped up & grabbed Amy, it was almost like he broke the starry-eyed spell Karma was under & that's what harshly jerked her back to the reality of what they were about to do & she bailed.