If it's just a little pink when the frogs aren't moving at all that might be okay. But if it was ever truly dark red, that's not a good thing. Maybe the frog's legs rubbed against something, but that sounds unlikely to me.

Red Leg is something caused by a fungus that can be lethal for frogs. It can be treated with an antifungal but you need to know the dosage. That's why I suggest a vet. Some experimenters have had success by raising temperatures for a certain amount of time, but that's not a sure thing.

It's probably nothing you did. Stressed frogs are more vulnerable to getting sick, but they don't have to be stressed to get sick. It doesn't matter how clean you keep the cage, (though a fairly clean cage is better than a dirty one). Mold spores can spread easily.