Sick Hen?

Picture of my girl who we’ve nicknamed “Nader” as she just mosies around in the ditch, pouring rain with tornado sirens haha!

She is a little over a year old and my free range girl, we had her a nice little fenced in area but she refused to stay inside so she’s just been free range ever since and comes back to the coop at night. She lays daily and has never had an issue before.

She was roaming around yesterday just fine but today I went out to get eggs and she was still in the coop (around 8am) figured she was maybe just laying an egg or staying inside later but now at 12pm she is still in the coop. No visible injuries but she just seems very lethargic, hanging her head barely lifting it, letting me pet her (this NEVER happens, she’s a stink).

Is it possible for a sickness to hit her that fast? I’ve never had a sick chicken let alone one act like this so I’m unsure of how to help her. Electrolytes in her water? Or what else can I look for to determine what might be wrong? Advice please and thank you!