Hi Babushka01, I feel for you. Since I posted that, I have spoken to her - fortunately a rare moment occurred where we were able to talk without her putting a wall up! We talked about how it works and why you get the high from it (nitrogen replaces oxygen so its a bit like being asphyxiated) - she said she doesn't do it a lot and has since sold her 'cracker'!
Any change in their behaviour is a worry and its difficult not to become consumed by it (I speak from experience here, got myself very ill a few months ago with the constant worry and stress of trying to stop her harming herself). We did pull her out of her school last year as it was part of the problem and things are a lot better now, she also had some sessions with a psychotherapist who was brilliant - took me a long time to find a good one but it was very worth it. And she has helped her to see things differently.
I don't really give my DD any money unless its for essentials, or she earns it by doing washing up, ironing etc - she does have a little job though and like you, I kind of want to limit her access to money so that she can't spend it on stuff like this.
I still veer between 'total lockdown' and 'freedom' regularly, its sooo difficult to try and walk the middle path but you know your DD best and if she is anything like mine, yes, total lockdown would mean that she would walk out and stop communicating with me which is what I fear most. So I try and give her freedom with boundaries, that way she knows that when I say she can't do something, I mean it. Doesnt' always go smoothly of course!
Is there anyone like an auntie or one of your friends that she is close to that would be able to communicate with her? one of my friends texts my daughter regularly and that has helped, I appreciate that sometimes (most of the time!) she doesn't want to talk to me and at least that's someone I trust. Could she go and stay with anyone for a few days? Could you take her and some friends to the beach or something? sometimes it helps to just distract them if they'll allow it and a different environment can help take the intensity of being in their bedroom on social media all the time. I know that my DD is so much worse when she's been spending a lot of time on her phone, she gets overwhelmed with all the info/images/criticism coming at her.
It can be really difficult to get to the bottom of why they might be acting differently but my advice would be to just try and keep talking, or texting when they're out just to try and keep the lines of communication open. hope things get better for you x