Are there any eggs in the box with her?? I found a broken egg under a broody the other day. She had it all over her. 
 
I would remove her from the box, put her on the ground and watch her for a few minutes. Wait for her to poop and check the poop. Broody poops are horribly stinky and large amounts. Watch how she walks and acts. Hopefully it’s just normal broody and/or molting behavior.
If I pull a broody from a box, she will usually sit still for a few seconds, all puffed up, but will then shake it off and go get a drink. Then move around and eat a little before trying to return to the box.
If an egg broke inside her, or if she has an egg problem, sadly i believe they go downhill very quickly. I’m not very experienced in that area.