The others aren’t having diarrhoea because they are not having digestive problems, whereas your poorly one has clearly got something going on. Please do give the others a wider variety of vegetables in their diet, introducing new veg slowly. Lettuce twice a week isn’t enough. They need vitamin c rich veg every day as they can’t produce their own vit c (Although hay does also contain vit c as well). Ensure their diet is mostly hay, one cup of a variety of veg per piggy per day, and then one tablespoon only of pellets each day (don’t let them have access to pellets all the time, they aren’t good for them)