Yes, that yellow stuff is chicken pus. She's been sick for a very long time, and probably was internally laying or infrequently laying. I had an unstarted pullet die of salpingitis, and when I necropsied her, found she'd been internally laying and had that same type of pus buildup inside her. Because she'd NEVER laid an egg, I kept holding out hope that she was just a 'late bloomer', by the time I became worried it was already too late for her. She too, had that skinny breast bone, which you'd never notice unless your hen allows you to pick her up and examine her. I want you to understand that reproductive problems are extremely common in chickens, especially if you get your chicks at the feed store (mass produced chicks for sale where all they care about is selling chicks, without a care in the world about genetics); more often than not, there is absolutely nothing you can do over the long haul if you happen to have a hen (or pullet) that is prone to reproductive disorders. What I'm trying to say is it's not your fault, and absolutely nothing you could do to prevent it... you did all you could for her, so don't feel guilty.
