When people have "insulted" you, it's been in response to what they see as you NOT caring for your child; for what seems like a quite determined absence of curiosity about what's actually going on in her head. You come across like you care more about your image of your child and what kind of person she is and your image of yourself as a perfect parent with a perfect family than you do about what's really going on with her as an individual, and you flatly reject any suggestion that doesn't fit the image. People can only go on what you've told them.
There are 4 basic possibilities here:
- She's started overeating because she's miserable. (Did you ask her if her eating habits have changed? I guess you must have done, during the conversation where she told you she wasn't happy about the weight gain. But you didn't tell us). Have you asked if she wants to leave boarding school and go to a day school? Ditto.
- She's gone on hormonal contraception and isn't telling you because she knows you'll lose your shit. Obviously there could be overlap between this and #1 (an abusive relationship, sex she doesn't want, etc). But the misery obviously could be for various other reasons.
- This is all to do with health issues to do with the surgery and / or the condition that led to the surgery. Maybe something went wrong and messed up something hormonal.
- It's health issues that aren't anything to do with the surgery (PCOS maybe, could be a few things) and that's just coincidence.
3 and 4 are the most likely; you need to get it properly checked out. But if there's nothing there, it's time to start looking closer at 1 and 2. But whatever it turns out to be, please, please be very careful to not let your daughter see your class-based snobbery and contempt towards fat people. (I mean, she probably has already, but you might be able to walk it back a bit).
What you don't have is an option 5: it's all the boarding school's fault and they need to "close the kitchen at night". Teens have pocket money and they'll buy snacks if they want; it's perfectly possible to eat 1000+ extra calories a day for about £3. They'll control their own portion sizes at the school meals too. And her freedom to decide for herself what she puts in her mouth will only increase as she gets older. You won't be able to call her university and tell them to feed her less. All teens have to learn to regulate this stuff for themselves, boarding school or not.
Talk to her (gently, kindly, not accusingly): about whether her habits have changed and if so, why. About whether she's been feeling hungrier since the surgery. About whether anything's making her unhappy. About whether she wants to change to day school. Act accordingly with the medical investigations. But just treat it in a matter-of-fact way, like anything else that's going on with a person that clearly isn't medically normal - like if all her hair had fallen out or something. Like: there's an issue here but we're going to consult the experts and get the right advice / treatment, and there's no need for embarrassment etc. No shaming; none of this "people like us don't have fat kids" vibe. She WILL pick up on that. Treat her gently. Good luck.