Hey bluefire,
Im sorry that your hen is doing poorly. Hopefully it was something she ate and she will feel better soon.
Washing the white poo off her butt is a good idea, however you dont have to give her a full bath if you think that it will stress her too much. Just hold her butt next to the running water and slosh enough water on it to get the poo off. Sometimes if it is really dried hard, you can snip a feather or two to get a big chunk of poo to fall off. Wow, that sound icky, LOL.
As d.k. said, she should have calcium available to eat free choice. You can buy oyster shell at the feed or pet store. If you feed eggshells back to her, make sure that they are crunched up small so that she doesnt recognize them as eggs or she may learn to eat her own eggs.... a tough habit that is rarely broken.
Some people just rinse out the empty egg shells after breakfast
and other people bake them in the oven before crunching them up. Up to you.
Sometimes I hard boil older eggs and then mash them up, shell and all, and feed them back to the hens.
Here is a link to a good informative site on chicken illnesses where you can look up symptoms:
http://www.msstate.edu/dept/poultry/diagext.htm
How is your hen doing now?
Chel