@Liddy
So sorry for your loss but a big WELL DONE for opening her up. I know that takes a lot of courage and gumption. It would appear that she was laying internally and developed peritonitis as others have mentioned. Internal laying can go on for weeks or even months and the hen behave relatively normally other than a slowly swelling belly which is generally well hidden by feathers. Sometimes a poopy butt will indicate that there may be a problem but sometimes not. The hen will sometimes start to walk with a wider stance and not be quite as agile due to the increased weight, but these are often not obvious until you have gone through an experience like this and become more observant because you know what to look for. I'm not sure why some become infected like this whilst others can have a belly full of egg yolks that are relatively inert and it is just the sheer mass of them that kills them, but there was nothing you could have done to prevent it.
@Liddy
As others have said, that is a lash egg which would be impacted in the oviduct and is caused by Salpingitis which is an infection of the oviduct. The infection is usually isolated to the oviduct and therefore ascites is unlikely to occur because the abdominal cavity is not compromised. Sometimes however the lash egg causes egg yolks to back up behind it and spill out into the abdominal cavity but more often it acts in the same way as being egg bound and constricts the gut preventing the bird from passing waste and they die of toxic build up. Usually a bird with Salpigitis will have an obvious swelling below the vent and a soiled and sometimes balding butt whereas ascites and internal laying tends to cause swelling lower down between the legs.
