Is this Coccidiosis? Lethargic and thirsty Belgian d'Uccle

[Polish rooster]

My Belgian d'Uccle, Chewy, has started exhibiting different behavior. On the weekend, she developed diarrhea, which she has had before, but this time seems more persistent. Her poops alternate between watery and full of discharge, very runny, and sort of well formed (see photos).

She still wants to come outside with me, but she seems lethargic and slow; not her usual pace. She is drinking excessively, and much less into treats and food. I see her next to the water all the time, taking large gulps. Her stance is also somewhat changed, with her haunches held higher than normal. She is less vocal than her usual chatty self.

At first, I thought some of these might be symptoms of stress and loneliness because all her flockmates are currently broody and she's alone all day (until I stop by and I pull them off their nests for a few minutes). She did seem to be in distress from this, walking back and forth from the coop all day calling out for someone. Could this stress have triggered her illness? A few weeks before this, she had black dots on her comb (fowl pox?) but those have healed almost completely.

I have Amprolium 9.6% solution handy (from a previous scare, which turned out to be false alert), so I started mixing it into the water today at 2 teaspoons per gallon. I omitted the vitamins and minerals I typically add because I read that it's counter-productive.

Is there any harm in continuing this treatment if this is not her ailment? What about the other hens, who are not currently experiencing this symptoms?

Chewy has not been vaccinated for Coccidiosis, only Marek's. She never ate medicated feed before. She is about 10 months old.

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Initially coccidia shows like foamy, bubbly lose poop. then is bright green and very watery as it becomes more severe. Green tends to be long and very skinny, watery part very wet and clear. The poop on the picture is muddy, dirty green and has white large discharge. I would check for crop issues. Check also the vent for white whitish "cottage cheese" discharge stuck to the feathers. If her crop is mushy and does not empty, and the vent has the whitish discharge you looking at yeast overgrowth (gleet) which can overwhelm the whole gastrointestinal tract and then even spread to the reproductive track. Normal whitish urate discharge is not runny as the one on the picture.
The muddy green poop suggests food does not move and stays in her crop for way too long so she is digesting and passing rotten food. To confirm massage the crop very gently and if she "burps" foul-smelly gas that is 100% yeast overgrowth. Needs to be treated with foods that suppress yeast growth if you do not want to go to a vet and get something like Nystatin. Actuated charcoal helps absorb some of the toxins produced by yeasts and the rotting food. You might have a combination of gleet and coccidia.
Lab tests are not really expensive and you can collect poop and send it to an avian lab in Georgia that specialize in poultry testing.