What does it mean when a girl pushes her tongue against her cheek?

Why would a girl do this if she doesn't have anything on her mouth?

Is this a gesture?

What would she be trying to say in this way?

I've seen multiple girls that I've crossed in the hallways and some of them are actually looking at me when they do this. They've basically use their tongue to push her cheek outward, making it look like a bulge in her mouth. Most of the time, they do it with their mouths slightly open but it makes it look like there's something big in their mouths (but again, nothing's in their mouths nor she's eating anything).

Their normal facial expressions are normal of them don't actually look at me but they know that I'm there (because I'm the only one they "see" when I'm close while they do this.

What does it mean when a girl pushes her tongue against her cheek?
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