Sadly I also have experience of bladder stone piggies.
Roxy our first sow to suffer with bladder issues never actually developed a stone, but her bladder was full of sludge and she had to have surgery to flush/clean it out. She lived for another year after her surgery, but we finally lost her when her bladder perforated and we sadly had to help her to Rainbow Bridge at the age of about 4 1/2.
Tetley had shown no signs of bladder issues until a couple of days before his symptoms presented. He had surgery to remove stones but sadly more had formed within a matter of weeks and he hadn't recovered well from the initial op so our vet at the time felt it was kinder to PTS - he was only about 3 1/2.
Curly was a huge rex boy of about 3 when he suddenly started to exhibit the usual symptoms of stones. An x-ray showed a large stone and surgery to remove it was successful and he went on to live a long and happy life and was never troubled with stones again.
My latest patient is an almost 5 year old neutered boar called Jazz. He was found to have a large stone lodged in his urethra tube. Sadly my vet couldn't get to the stone so took the decision to cut through the tube to remove the stone and then she formed a stoma to allow Jazz to wee. It is early days but so far he is coping remarkably well, we are about 3 weeks post op. He is still on Septrin, but is eating by himself and has made a very good recovery from the surgery, our only worry is whether the stoma is going to hold up long term.
Smudge is about 6 years old and she has been troubled by bladder sludge and stones for years. So far we have avoided open surgery to remove the stones as my vet has been able to extract them or flush them through. She is currently on Zithromax (antibiotics) for the first time to try and clear up a stubborn infection and so far is responding very well.
I can't tell you what to do, but I would strongly recommend x-rays to see if any stones are present. If the stones are small enough your vet may be able to remove them with forceps/flush them out. It may be that your piggy has an on going infection in which case a longer stint on antibiotics may be worth a try - the usual AB of choice for bladder issues is Septrin. In Smudge's case she has been on 0.7ml twice daily for up to a month in the past. I also found that by giving her Metacam (pain relief) to help her through the first few days made a huge difference as she would continue to eat and apart from squeaking when weeing/pooing didn't seem unwell.
It is definitely worth looking at diet/water hardness etc. We are now filtering all our water and have been avoiding his calcium foods for several years now. I am also looking into introducing cistease to my stone piggies. I used to feed Cavy Cuisine pellets but have read on Guinea Lynx (an American forum) that bladder stone piggies tend to do worse on CC so we have switched back to Excel pellets, but only a small amount daily and I have introduced more mixed hays which the pigs love.
Good luck and keep us posted.