Really late update, but she died later that day. We were lucky enough to figure out the issue- she had an egg break inside of her which I am told is called egg binding syndrome. Thank you all for your help and sympathy.
1) What type of bird , age and weight (does the chicken seem or feel lighter or thinner than the others.)
2 Year old Rhode Island Red. Normal weight.
2) What is the behavior, exactly.
Found her lying in the mud, not moving. When picked up, she flaps, but can't lift head, stand or move...