I've seen videos of doctors wearing multiple masks and pulse oximeters to show they still have plenty of oxygen. Here's one I googled where the doctor is able to breathe fine through 6 masks layered on top of each other.
https://www.youtube.com/watch?v=k5npQivAmcE
I imagine it's just that it's uncomfortable and awkward so she's getting herself worked up over it. I am highly suggestible, myself, and can easily talk myself into concern over a problem that isn't really an issue. In my experience, kids are even more prone to this tendency.
I suggest you either get a pulse oximeter (available at any pharmacy/pharmacy department) or take your child to a clinic/doctor's office and have her O2 levels checked while wearing her mask. I am confident that the mask isn't actually impairing oxygen access. In the unlikely event that her specific mask type is the one exception that causes breathing problems, you can switch to any other mask.
Fwiw, my Dd used to complain every fall when cooler temps meant that she had to switch from shorts to jeans that the jeans hurt her. She'd try on every pair of (identical) jeans she had, only to decide the last pair was wearable. The next day the routine would be repeated, with the only difference being that the pair that was finally deemed acceptable had been rejected as intolerable the day(s) before.