Ameraucana not laying yet, but won't leave nesting box

[wab7]

I am totally new to the chicken keeping world. I have one Blue Wyandotte and two Ameraucana. The Wyandotte has been laying since early June, the Ameraucana not at all yet (I think they are around 24 weeks old). The problem is that two days ago one of my Ameraucana's starting sitting on the Wyandotte's egg and will not leave the nesting box. She is there all day and night. Yesterday I picked her up and put her out of the coop. She was very passive - not her usual behavior - but she walked around a bit and got right back into the nesting box. Should I be worried? Can a chicken who has never laid an egg be broody? Might she be egg bound? I am not sure what to do.

 

[Happy Chooks]

Is she puffing up at you? Is her chest plucked bare? Does she make any soft clucking sounds? If yes, she's broody.