Well just tell someone later, when your mum is not with you. Yes, it does matter, because some things change when you smoke. If you were up for an operation you could be needing a higher dose of anaesthetics. When getting things done in local anaesthetic dito. So the question does make sense, but I can understand you not wanting to tell in front of your mum. Can't you send her out? If your over 14 that should be possible at your request, even if she might be a bit annoyed, but she could talk to the doctor afterwards and you can tell him/her not to tell about the smoking bit.