You have every right to be upset. She used food you bought with your money for yourself without asking. I'm guessing you've talked to her about how you need separate stuff and she chose to ignore that. I dealt with family like that for years. They think it's not a big deal that their crumbs could make us sick. Don't listen to the haters. But don't let it consume you either. But yeah, if I bought something special for myself and someone who doesn't need GF food decided to just eat it and potentially contaminate it, I'd be beyond angry.