Hi!
Give the probiotic either one hour before he antibiotic (US recommendation) or 1-2 hours after the antibiotic (UK recommendation).
It doesn't matter which way round.
If your piggy is really off its food and you have a healthy companion, the best thing is 'poo soup' made by soaking healthy poos that have just just been dropped in a little water and then syringe the water. This ensures that the gut microbial gut biome is getting into your ill piggy's digestive system still alive and restocks it with exactly the 'right stuff'. Put the healthy companion in a carrier or on a towel covered chair and feed them a little veg; this usually stimulates poo production.
If absolutely freshly made, it is much more effective than probtiotics and it also mimics natural behaviour of recovering piggies even if it is a bit gross.
Probiotics, Recovery Foods And Vitamin C: Overview With Product Links