The thing is, it's not the cancer that's made her like this - she was miserable before the cancer, but it is just giving her even more opportunity to play the martyr. 'I can't have a double because I have to work to feed my family', 'I'm all alone, no one understands'...etc, etc
The scene last night where she went into the fit club and the women were joking, completely unrelated to her, and used it as an excuse to turn it all about her and bring them all down was so her.