I'm pleased to report that Toots is around 75% 'normal' this morning. I put a huge pile of fresh hay in their enclosure and she dived right in! Then I put a big bowl of fresh parsley and dill down and she stuck her head in and hasn't yet come up for air!
So Debbie - looks like you were right about some piggies taking a turn for the worse for a few days. Not something I've experienced before as she was in top form, even after she was spayed at a year old, so I've learnt a valuable lesson. Maybe the older they are, the harder it hits them? Thank you everyone for your helpful comments. Piggy people are the best!