Hello. Soft poos while on antibiotics are fine, especially if you are giving her probiotics to keep her gut working well. Diarrohea is a common side effect of antibiotics in humans too - especially the penicillin derivatives. So don't worry.
I have given part of a weetabix, or a bit of all bran as a help in the past, (different piggies liked different cereals) but it can help as it gives extra fibre. The only other thing is to make sure she's drinking plenty and maybe give a small piece of banana as a treat because it is a rich source of potassium and piggies with loose stools can lose potassium.
If she does move from soft stools to genuine diarrohea then please take her straight back to the vets. They can dehydrate very fast indeed. But soft stools just show she's reacting to the antibiotics really...
I hope this helps a bit, wishing Connie a speedy recovery,
Sarah