Praise be to Allah.
You have to pay attention to her condition and not do anything that may make her illness worse. If she cannot bear the smacking, then it is not permissible for you to smack her. But if the illness is minor and she does something wrong or does something for which she deserves to be disciplined lightly, then there is nothing wrong with that.
But you have to pay attention to her condition. If the smacking will harm her, then do not smack her. But if the smacking will not harm her because her sickness is minor and there is an urgent need to discipline her so that she will stop doing something that is inappropriate, then it is OK.