She was meant to be sick?
Admittedly I don't know much about late-stage leukaemia but surely by the time the cancer has spread to other parts of your body, you're probably not going to be able to be gallivanting around all day without any problems? Certainly that's true of the people I know, though I realise you can be "dying" for a long time.
For most of the movie this girl looked very well (not particularly pale, frail, no dark circles etc.) and was running and walking around everywhere! She made references to being tired a few times and sometimes panted but she constantly ran fast away from people, walked everywhere, climbed a tree, ice-skated without any apparent muscle pain. She was also able to drive easily, and was not very skinny. She fainted twice and had a nosebleed but then she'd just be up bouncing around again. Obviously she got worse but even so, I didn't think she seemed particularly ill for most of the movie which seemed unrealistic.