Young pullet slumped forward, not sure what's wrong

[Mofish]

I have a young 2 month old pullet that has been slumping forward with her wings dropping down for the past 2-3 weeks, but I'm not sure what's wrong with her. She still eats feed and drinks water, and does try to forage and scratch, but her legs seem a little weak and unstable. She also feels a bit on the thin side. She is unvaccinated. I live in Louisiana, so it is very hot here, but she has shade and plenty of airflow in the chicken tractor. Not sure if this is Marek's, heat, or something else. Please help - what is wrong with her?
She is the chicken on the left in the photo.

 

[MysteryChicken]

Might be heat stress, I had a welsummer cockerel like that. I gave him save a chick vitamins and electrolytes, & he perked right up.

 

[Mofish]

Thanks MysteryChicken!
UPDATE: Since Friday afternoon I have brought her inside and kept her in my laundry room and gave her some electrolytes. She has gotten stronger, but still goes "slumpy" and doesn't hold her wings to her body a good portion of the time, it's just not quite as dramatic as the pic above showed.
Other observervations: I noticed that about 5% of her poops are small and black (not as "fluffy" as healthy poop). I also notice her showing some redness around her face, although I'm not sure if that's just her maturing. She's almost 9 weeks old at this point.

Any other ideas/suggestions??