Hi
Please have your piggy checked whether there could be other issues like bladder/kidney stones or sludge to make sure that that angle is covered.
There is a condition called sterile cystitis with regular flare-ups that is not much known outside vet circles that see guinea pigs regularly. It is characterised by a zero or low count of bacteria (which is why it cannot be healed by an antibiotic and can - in milder cases - only be temporarily suppressed). Most guinea pigs do have some faecal bacteria in their urinary tract since the tiny scent gland for territorial marking is located right in front of the genitalia (too many of them are responsible for UTI), so a low bacterial count is more common.
The condition is similar to FSC (feline sterile cystitis) and management in guinea pigs generally follows treatment for cats, mainly by higher long term doses of dog strength metacam and by a glucosamine supplement (like for instant cat bladder capsules; the contents from 1 capsule mixed with 2 ml of water and then syringed either once every 24 hours or 1 ml every 12 hours) since it seems to affect particularly the natural glucosamine lining of the walls of the urinary tract that prevent highly corrosive urine from coming into direct contact with raw tissue. The glucosamine and metacam will take a few weeks to build. After that, there are going to be flares every few weeks for which you have to work out just how much you can safely up the analgesic and the glucosamine for a few days to get the acute symptoms under control quickly. Sterile interstitial cystititis (or sterile IC) runs the whole gamut from the very mild to the very severe. Medium and severe cases can now be treated by cartrophen (in more severe cases in combination with glucosamine). In milder cases, the condition can eventually disappear on its own, usually more in a matter of years than months. Healthy piggies can fend it off with their immune system without ever showing symptoms.
The condition seems to be particularly affect guinea pigs (and cats) with a nervous disposition and has been on the rise for the last 15 years or so. It is still usually diagnosed by default after all other possible issues have been excluded.
Links - Interstitial Cystitis - Guinea Lynx Records
I hope that this will give you and your vet a new avenue to pursue. Milder to medium sterile IC once under control is more in the way of a ong term nuisance as long as you can work out how to get on top of the flares quickly after the initial build up. Please be aware that new foods, like seasonal fresh grass, can sometimes trigger a flare but that on a good normal balanced diet sterile IC does not cause stones or impact on the overall life expectancy. You are thankfully not dealing with any of the severe forms where an antibiotic has not impact whatsoever and the decline is very quick.
I would however recommend to invest in some cheap kitchen scales for a weekly weigh-in and life-long health monitoring in conjunction with a body-onceover. More importantly, regular weighing your biggest ally in managing illness and especially loss of appetite. Unlike poos - the output of which is always running at least a day behind the food intake - a daily weigh-in first thing in the morning for best day to day comparison will give you instant feedback and help you spot developing issues early and work out the appropriate level of fibrous feeding support each day in order to get a piggy through a crisis together with the necessary medical support.
Weight - Monitoring and Management
How To Pick Up And Weigh Your Guinea Pigs Safely (videos)
