Well the folks at Backyard Chickens came through for me, but not before my jumpy girl tried for hours to jump out of the isolation bin.
It seems that what I was seeing was not a disease, but an outie... that's right, all birds have some sort of belly button where the umbilical cord connected to them in the egg, but some have these bloated parts. Apparently they fall off in a few days. The main concern is that the area is pecked and becomes infected, which can happen during the pecking order ritual.
So, when I had the Welsummer in isolation, she and the other two were very unhappy. The two "healthy" chicks braved the brunt of the heat lamp to sit shoulder to shoulder up against the wall closest to the quarantine bin, while the Welsummer was inches away in the her bin. The chicks were VERY vocal that day, and very unhappy. Seems even after a short time together, they didn't like having their flock separated.
So, after reading all about the situation from the folks at BYC, I had to decide if I wanted to risk putting them together. One comment suggested putting one other hen in with her for company, but that would leave my remaining chick alone. Having three is not a good situation right now. Hehe. So, cautiously I put her back in the brooder.
The odd thing is that after just one day alone, she seemed significantly smaller than her sisters.
One thing I've noticed in my few days as a chicken wrangler, is that they don't seem to like to eat out of their tray. They much prefer to hunt and peck. So I've taken to sprinkling a bit of food on the ground, which is 50% scratch. They seem to like eating that way better.
So, all is well in chickenland again!