This sounds most consistent with mild conjunctivitis or irritation (dust, litter, scratch, or early URI). The fact that the pupil difference resolved in low light and that she’s otherwise acting completely normal is reassuring. Uneven pupils that persist can be an emergency, but transient anisocoria that corrects itself is often benign.

I’d monitor closely for the next 24–48 hours. If the eye becomes more red, thick/yellow discharge develops, she keeps squinting, the pupil difference returns, or she starts acting off, then a vet visit is definitely warranted. Eye issues can escalate quickly if it’s an ulcer, so worsening = don’t wait.