Suddenly Lethargic/Loner Hen: Concerned about the rest of the flock

I have an 8-ish month old Buff Orpington who has been displaying the same loner/lethargic behaviors for about 2-3 days now. When I opened the coop this morning, this one just stayed in the coop, laying down. She eventually did go out, but has stayed near the coop, either laying down or just standing in one spot, which is unusual. I went out a little while ago and she was standing with her head down.I have also noticed an increased aggression in the other chickens (another BO, and 2 Barred Rocks). I do not believe she is egg bound because she is just laid a normal looking egg last night/evening. We have separated her from the rest of them, but since I don't know what she has, it is hard to know what to do.

One of the BR's has soft stool, light brown in color, and the BO in question has runny, white and green stool (no blood). I checked her crop, and all seems normal.

We had something almost identical happen about 2 months ago to our Americauna pullet... eventually she had what appeared to be a seizure and died. I am concerned that they are connected.

Last night when we separated the sick hen, we cleaned all of the bedding out of the coop, and sprayed it down with a hydrogen peroxide solution to sterilize it, prior to putting in totally new bedding.

The local feed store has suggested antibiotics as the only course of action.

Basic Info:
1) What type of bird , age and weight: Buff Orpington, 7-8 months, more than a pound?
2) What is the behavior, exactly: lethargic, keeping head down & neck in, not socializing, rarely leaving the coop- but still mobile.
3) How long has the bird been exhibiting symptoms: 2-3 days
4) Is there any bleeding, injury, broken bones or other sign of trauma: NO
5) What happened, if anything that you know of, that may have caused the situation: Nothing known, other than previous hen doing the same thing several months ago, resulting in death.
6) What has the bird been eating and drinking: Layer Crumble, and some scratch.
7) How does the poop look: watery, white and green. (Other birds normal, and 1 with soft/brown stool... almost but not quite diarrhea)
8) What has been the treatment you have administered so far: about to start Antibiotics
9 ) What is your intent as far as treatment: treat myself
10) If you have a picture of the wound or condition, please post it: N/A
11) Describe the housing/bedding in use: Homemade pine coop with pine shavings... 4-6" of shavings.