I'd say continue the Fastrack and maybe also add some Stomach Soother. I've got a horse on antibiotics after surgery and he went off his grain for a day or so. Gave him some Stomach Soother and now add it to his grain and he is back to cleaning up his feed. It really seems to help with tummy troubles.
When I do antibiotics, I try to dose a pro- and prebiotic as evenly apart from the antib as I can. So, if the antibi is at, say, 8am and 8pm, I *try* to to the pro/pre in the middle of the day, and then last thing at night (as I'm not going to get up at 2am ). Or, if things are just hard on the horse, a couple hours after the first dose, a couple before the 2nd, and then a couple after the 2nd.
______________________________ The CoTH CYA - please consult w/your veterinarian under any and all circumstances. - ET