If your piggy is suffering with soft poops, then you need to remove all veg and grass from their diet completely and feed only hay (and the normal one tablespoon of pellets) until the poops go back to normal. Once things have gone back to normal, keep the off veg for a further 24 hours and then you can start to slowly reintroduce veg into their diet. Starting with herbs is advisable as they tend to be kinder to the tummy. You can then slowly rebuild their veg diet, monitoring for any soft poops and stopping veg again if the poops go soft again.
Do remember that they should only have one cup/50 grams of veg per day.