Margot The problem isn't you, it is her. Some people do not want to be satisfied, they are happiest when they have something to complain about and, regrettably when people are ill, but not seriously, they are at their worst.
Should she be in hospital again and if you visit, and I would not blame you if you didn't, you can enquire about her illness and if she complains and says she desn't want to talk about it, just say you were cncerned because you understood she thought you were uncaring because you didn't ask about her illness after she said she didn't want to talk about it. Put her kindly on the back foot. What is more, if she complains about you again just laugh and tell the person telling you that she is up to her old tricks again and she did that last time and she is just one of those people who is never satisfied.
Or better still, just do not visit her.