Hi!
Your piggy is very likely suffering from a sterile institial cystitis (more commonly known in piggy circles as IC). It has become rather common in guinea pigs over the last decade. I currently have one of them myself with the same symptoms and problem but I have had piggies with it before. Because it is sterile, i.e. non-bacterial, antibiotics don't work and can at the best only temporatiry suppress symptoms.
Unfortunately it is very much a default diagnosis after all other potential bladder/urinary tract issues have been ruled out. Since nobody knows what is causing it, it can currently not be healed and any treatment is only for the symptoms. The good news is that it is not life shortening, does not cause bladder stones and that it eventually goes away on its own. The bad news is that that is generally a matter of many months and years.
My Nerys had IC (after being treated for half a year for chronic UTI without any result) for about 3 years but then lived another 3 years totally free of any urinary tract problems to the ripe old age of 8 years. My Caron had a milder form of IC for ca. 1 1.2 years and lived the same amount totally free of it as one of the longest lived of her huge rescue intake from horrendous circumstances. That gives you about the time frame within which you have to think. It is not pleasant at all, but it is not a killer disease.
As to treatment:
- Metacam (loxicom, meloxidyl etc.) is used in both aspects as a painkiller and analgesic; this can be given constantly with the dosage upped during an acute flare up. Thankfully unlike other species, guinea pig kidneys cope with it well, even in long term use with IC or arthritis.
- Cystease or a comparable glucosamine based cat food supplement (it is not a medication and capsules are easily available long term) are recommended to help support the natural glucosamine coating of the bladder walls. It makes a difference even if you do not necessarily see a huge chance in symptoms; but you will know instantly if you stop or go too low with the dosage.
- A no-grain, no root veg, no lettuce and some say even no grass diet can help remove trigger foods.
You can find the recommendations in our diet guide:
Long Term Balanced General And Special Needs Guinea Pig Diets
Here is some more information on IC, but your vet may want to do his own research or contact an exotics vet with an interest in guinea pigs.
Links - Interstitial Cystitis - Guinea Lynx Records
I hope that helps you and gives your vet a new avenue to pursue?