Today I found one of my chickens (13 weeks old) lying on the ground and not able to get up. She was awake but as the other chickens walked on top of her legs, she couldn't move her legs to get them off or even complain as they "trampled" on her. I picked her up and gave her a warm bath and visually inspected her but found no signs of trauma. She definitely favors her left side/leg because when I put her down on her left side to try to wash her right side, she immediately flipped over and flapped her wings in protest.
She is eating but not really drinking. I made a mash so that she could get some fluids. She hasn't pooped since she has been quarantined so I haven't seen her poop to inspect for worms of if her poop is abnormal. As I was rinsing her off, I thought I noticed a very runny poop but wasn't sure if it was just the "wash off" (she was literally lying down in some poop when I found her).
Here is a video of her in her quarantine space:
Her "hurt leg" twitches sometimes as she tries to move it. However, generally speaking, she doesn't move it. Also, her toes are curled on that leg (similar to chicks that take too long to hatch and need booties). However, she has always walked around without any problems and developed normally.
I also inspected the coop and couldn't find any evidence of predators getting in.
Any ideas? Worms? Lice? Mites? Is that the standard check first? I just wasn't sure if they would cause her not to be able to walk. Something else? A disease? I have no idea. I have to do more research. Thank you everyone.