Comb and wattle receded

[RubelliteRose]

Should I isolate her from the others?

If she isn't being picked on and if you don't feel like you need to closely monitor her, I would leave her with the others. Separation would be stressful for both her and your other chickens.
I thought about egg binding, but I would think that her comb would be red even if this was the first egg in a while. Maybe she has developed ascites?
I will tag a few of the BYC experts to see if they have any ideas for you. @Eggcessive @Wyorp Rock @azygous

 

[Eggcessive]

I agree that I would keep her with her flock, but watch out for any pecking or bullying. Some may sense that she is not well, and pick on her, but not always. Being with the flock would be the best thing for her. Tail down and the other symptoms sound like she may be having a reproductive or other disorder. Make sure that she eats and drinks daily, and check her crop in early am to make sure that her crop is emptying. Crop problems sometimes accompany reproductive illnesses. Does she have any weight loss or an enlarged lower belly? What do her droppings look like?

 

[UKchooks]

I haven't watched her pass anything, but haven't seen any unusual droppings in the coop or run. Her crop seems to be emptying fine and she's eating and drinking well. Not sure about weight as I hadn't been handling her much prior to this as she's usually very speedy and feisty! But it does look like her belly might be somewhat swollen.