Oh, I can understand how this can bring a lot of anxiety! I started doing this at 7weeks with my babe to teach him how to safely get things off of his face because I was scared of the same thing. Take it as an opportunity to teach with a game! They love playing and learning new skills. I would toss a thin blanket over him and when he lifted his arms, I would help pull it down as he lowered his arms (to learn how to pull off the blanket) Same goes with safely getting off the couch or bed or chair.
It's a pretty common thing for them to do at this age. Although mine is usually in onesies so he can't pull his shirt over his head but if he's near his blanket he'll do that.
My son is 11 months old and is doing this but he does it when he wants to play peekaboo with us, when he can't breath he moves it, if she knows how to move the stuff off her face then she should be okay You probably reacted, not even a laugh or anything but just reacted to her doing it once so now she finds it funny
It's scary at first but with time you'll get more comfortable with it.
How old is she , because my 6months old does this