I agree with vicky, it sounds like a soft egg has broken inside her. You could have a go at trying to remove the mess with warm water, then don some surgical gloves and very gently feel inside the vent and remove any egg and shell you find. Egg binding can be very serious and hens can die quite quickly from it. It's important to get her cleaned up asap and the vet may well prescribe antibiotics if the mess inside her has got infected, which is quite likely. Good luck at the vets.

__________________

Dwell simply ~ love richly