Any time you notice a chicken is not acting quite right, it's a good idea to isolate it. You can rig up a hospital cage out of many things: we use an old dog crate with some pine shavings inside.
Isolation provides many benefits. First off, if the chicken actually has something that's contagious, you may help to prevent its spread to the rest of your flock. Next, a chicken who's a bit under the weather often gets picked on by the rest of the flock, which is stressful for it and makes it harder for its immune system to fight off whatever is making it feel under the weather in the first place.
Next, the chicken keeper is better able to observe a bird in isolation. You'll be able to observe its droppings better and monitor what it eats and drinks. And an ill bird will often benefit from being provided a little extra warmth, but if you do use some kind of bulb or lamp be sure the hospital cage doesn't get too warm and the chicken has a way of moving away from the heat.
Personally, I try to avoid making changes in diet when I'm observing a bird for potential illness, because this can complicate the picture. Sometimes a diet change can actually cause symptoms (changes in droppings) and you want to know what you're looking at.