She is probaby either trying to lay an egg or has laid an egg...either way she's staying still hoping thinking if she does, you won't see her. Hen's hate to be messed with if they're getting ready to lay or are sitting. They think their camouflaged if they stay real still!
I have a couple nest boxes up on the deck that are used by certain hens. If they are occupied when I walk by the hens usually hunker down as if I won't see them; but they eventually come out when they are done doing whatever it is they think they are (sometimes they don't even lay). These boxes are sometimes in hot contention and 2 or 3 hens will get going early in the morning when another is sitting where they want to be. They sort it all out, but not without a ruckus.
Now, the hens in the coop boxes always give me the stink eye when I find them in there and will frequently puff and/or cackle at me, but they are all pretty harmless...even the 2 broodies. They may peck once and look imposing, but they let me reach right under them without much fuss. I've never seen one of the hens in the coop boxes hunker down like the ones up on the deck do. I find that interesting.