The way I use probiotics like Fibreplex or bio-lapis is actually a bit more frugal and it's partly because my pigs don't like it straight out of the syringe so I mix a tiny bit into some Critical Care (we use the papaya flavoured Fine Grind CC) and let it stand around a few hours so the yeasts multiply up a bit (it's just basically yeasts in there to bulk up the gut microflora poulation). We use syringe food as part of the bloat treatment for my Louise - she's mad for the stuff and we need food to go in the top end and push the gas through. Louise doesn't lose her appetite until she bloats right up but she's been 'puffy' on and off for about 2 weeks now - so she doesn't really need emeprid (a medication which stimulates the gut to move) until her eating drops off - but I give her a little probiotic-in-CC mix each day. With her we think it's the springtime grass being a bit rich - it happened the same time last year.
There is a link here to some probiotic info including how to make a'poop soup' from her healthy companions
Probiotics, Recovery Foods And Vitamin C: Overview With Product Links
So did you find out about the stones 5 months back when all this started? Did she have urinary symptoms? Was she treated for UTI? Does she still have any symptoms of squeaking while peeing, blood in pee or incontinence/wet bottom end?
If she had antibiotics and they wiped out her gut flora she may just be battling to get back on track in which case some poop-soup water and Fibreplex in a bit of Recovery food (she might just eat it off the spoon if she likes it) could sort her out over a week or so. But I'm tempted to think there might be that there's nothing wrong with her gut flora - if she's carrying stones she should really be on daily metacam for pain relief and anti-inflammatory - and at a decent dose. Pain can make their eating intermittent and it could well be that she has days where she just gorges on veg because she can't face hay and this is causing an erratic digestion. My George has been on a decent dose twice a day for two years - he's also currently living with a big stone (and its little friend) but he also has arthritis. He's 6. George is currently on
about 0.35 ml twice a day and it's the stronger 1.5mg/ml dog version. And he loves it - it's not a chore to get him to take it.
I won't go into the ins and outs of stones here but if you had a copy of your x-ray and wanted to add it so we could get a squizz and see how big they are etc it can be useful. George's large stone bothers him when his guts move after the morning veggies so not eating could be related to her stones in that way.
A bit of daily probiotic (esp in the form of a poop soup) would be a cheap harmless way to support her gut. But have they given you any metacam or loxicom? If not this could be one way forward.