May I jump in here and ask a question? I hope you don't mind......but its a similar problem.
Several years ago, 2 of my hens developed a really bad infestation of mites, especially around their vents. I treated them with some spray and DE in their bedding. The mites went away, but one of them developed a swollen hard knot underneath her vent. I didn't do anything about it because it seemed to not affect her......but she was weakened after that. I'm thinking she might have gotten an infection there, and there was an abscess in there all that time. Is that possible? And that's why she was so weakened?
I eventually put her back with the others because I thought she was recovered enough, but within minutes they were trying to kill her, so I removed her immediately. Unfortunately, when I went back a few hours later to check on her in her crate, she was dead. I felt so bad. Anyhow........since some of you mentioned that infections tend to be hard, knotty things.........I'm thinking she had an abcess all this time and I didn't recognize. Thanks.