When did Kat got possessed? Here is my theory.

There are theory of Kat being possessed before the movie start and I was one of the people who supported that at first based on the smile of Mr. Gordon office, but after reading the interview with the writer (looks like he was trying to show how hard is to lose parents) and re-watching the movie, I think she got possessed the night of 22th Feb when Rose left her alone and she was dealing with the vision of 21th morning of the car accident. since she was expecting her parents show up morning of 22th and she called her father at the night of 21th but the devil answered her instead and told her "they are not coming and to kill the Nuns". not to mention, she was able to tell the grace at dinner the first night with rose. but she was not able to tell the grace at the 2nd dinner which support that she was possessed 2nd dinner and not 1st dinner scene.The scenes that support what I mentioned later, if you look closely at the reflection of the shower wall at min 01:00:30 you will see the demon. and the 2nd scene the demon showed up is when Kat was taking with Rose at the night of 22th before Rose leaving her alone. we saw the demon from Kat view of the camera min 01:04:00, not sure if Kat was seeing the demon or not ( I think not) but I believe the demon was waiting for her, but she was still not possessed.

So the timeline will be:
- Morning of 21th Feb, Kat see car accident vision, then at the night she taks a shower and we see the devil in the reflection, then she calls her father but she get the devil answer instead.
- 1st night of Kat with Rose 22th Feb, They do the grace and eat the dinner then Rose scare her with the sister story and leave her alone to go with her boyfriend. when she return, she finds her on the boiler room which looks like she got possessed there since we see her scorpion body movement when she sleeps :)

- Morning of 23th of Feb she can't do the grace and we saw that she was possessed with the demon voice when the Nun checks her but they get interrupted by the call of her parent accident, then she gets the call from the demon and start killing everyone.

So everything would make perfect 100% sense if only there was no smile on Mr. Gordon office. probably the writer put that smile just a cheap trick to trick us and puzzle our mind and open more theory about the movie.