Just to add - Stones can not only be caused by too much calcium (or an imbalance in the diet that upsets the calcium ratio in the diet/ uptake), but also too little!
Sounds mad, I know, but if there's too little calcium in the diet the body can leech it from the bones.. And then have the genius idea of excreting it, through the urine.
So be wary of going too far in either direction when it comes to calcium levels if the diet.
Girls are far better built for passing stones than boys, although I imagine they're still not best pleased about it (so far haven't had that experience, although sadly I lost my boy a few years ago to a stone/ poorly trained butcher, and one of his descendents follwed a similar fate on his second stone op last year

). Stick with vet advice for the most part, but NEVER be afraid to question their judgement and seek a second opinion if you are at all unhappy at the way things are going.
I believe (and having discussed this with a fair few vets, it seems true) that Septrin is by far the most effective antibiotic for the vast majority of bladder infections. And also, a piggy that is prone to cystitus will find it flares up again from time to time. Cranberry juice, pearl barley water, and plenty of fluids can help, but I'm wary to suggest flooding in an attempt to flush if this is a stone problem as you're speeding up the buildup of urine if there is a blockage. You know your pig, and if you're happy to treat as an infection then by all means, but an x ray might be the way to go (anaesthetic issues depending of course!) so you have a higher chance of the right diagnosis.