I'm really sorry for it being so confusing! i didn't realise it and expected the confusion to be more about Amber's feelings. I'll be more careful next time! I'll provide a more detailed background to the confusing conversations!

I kinda delibrately not give a name for Amber's boyfriend. He is supposedly just Amber's boyfriend. To make things clearer, i shall call Amber's boyfriend 'the jerk'. The scene first starts out with Amber walking back home with Kris beside her. Amber then starts sharing to Kris about how her dreams are all coming true. When Kris said that it was a coincidence, it was clear to Amber that he didn't believe her. So she went on talking about how another dream of hers came true. 

Of course, Kris still didn't believe it until Amber mentioned the whole cheating thing. Amber said that she dreamt that her boyfriend cheated on her with Kai's girlfriend. (Again, Kai's girlfriend is only just known as Kai's girlfriend.) 'The jerk' had cheated on Amber and was seeing Kai's girlfriend. This all happened without Amber or Kai knowing. I hope this clears up a bit of your confusion. 

Though it wasn't clearly stated, Amber does have some feelings for Kris long before she met her boyfriend and that feeling never really left even after she started dating 'the jerk'. At that time when Amber had just met 'the jerk', Kris had pushed her to him, thinking that 'the jerk' would make Amber smile more. They had started dating then. Kai is a mutual friend of theirs so he had brought his girlfriend along during one of their outings. Kris began to suspect something when 'the jerk' and Kai's girlfriend seemed to act really different. After confirming his suspicions that they had something going on, Kris tried to stop them and kept it a secret from both Amber and Kai. At that time, 'the jerk' and Amber had been dating for a while already.

That was why when Amber mentioned her latest dream, Kris was very shocked because he knew that it is true. From his expression, Amber kinda had the idea that Kris knew it from the moment the two had started going out, which was why she had accused him by saying 'and you knew about it." That is also exactly why Amber had blamed Kris for always pushing her to him. The whole "If you didn't push me to him. If you had just kept me to yourself, this wouldn't have happened" was something that Amber had blurted out. That statement kinda reveals that Amber blamed Kris for pushing her to 'the jerk' who she never really truly loved. It also reveals how Amber kinda feels for Kris. 

When Kris hugged Amber, it was meant to show that Kris actually returns Amber's feelings. That he actually feels the same way as her. He also felt apologetic for making her go through the unnecessary betrayal. Amber understood the hug which was why she pulled away from him and chose to go back upstairs to settle everything once and for all. Amber should not go to Kris without ending it with her boyfriend. When Kris understood it, he let out that oh so handsome smirk of his.

As to why Amber's feelings can change so easily, it's mainly due to the fact that no one can resist Kris. :P