I think this is it. Maybe she thinks that you should not buy luxury items because you are still at home since you can't afford a place of your own.
Enjoy your bag but perhaps don't splurge again before you move out and are truly independent.
(I'm 35 and still hide stuff from my mom, although there is really no reason to, except that she would give me a look. You're not alone )
I agree with this too. She's probably wondering why you're spending $800 on a bag when it could go towards a place of your own which I think is valid if you're still living at home.
I'm 34 and own the house that I live in and I still hide stuff from my mom too but it's not because I can't afford it. I just don't care to hear what she has to say so I don't use any luxury items when I'm around her.