Might you take a look at her bum maybe she has a cut or a bump or there is something there causing it.
Feel around to see if you feel anything, yes a egg to. you didn't say she has passed the egg I am assumming she has if not she could be egg bound in this case you will need to act fast. I know this will sound gross but you should feel the egg when you feel her bum but if not it could be back further, lube your finger (any long finger, veggie oil, vasiline) and put it in her bum see if you feel anything egg, lump, you know if you feel soumething that is not to be there yes I am sure you will know it. Don't feel anything? no swelling? any marks of any kind?? what colour is the poop? does she eat and do chicken things?
I don't know maybe some one else can help you.
Rhayden