Hi!
Your piggy could be suffering from a sterile interstitial cystitis (IC), which has become a lot more common over the last decade but is not necessarily well known to general vets. It is good that your vet has checked for stones/sludge because that is step 2 in the elimination process.
Unfortunately, as we don't know what is causing IC, it can only be treated symptomatically but not be healed as it stands, and it can only be diagnosed by default after excluding all other potential urinary tract problems. It does not much or at all react to antibiotics and cannot be cleared by them.

The good news is that is not causing any bladder stones or sludge and that it doesn't shorten life expectancy.
The bad news is that it is going away on its own eventually, but rather in a matter or years than months.
If it is any consolation, I currently have an IC piggy myself again.
Metacam and cystease (a glucosamine-based cat food supplement to help protect the natural glucosamine coating of the bladder walls) are the main treatment on a permanent basis. During acute flare-ups, you up the dosage.
Here are our diet recommendations:
Long Term Balanced General And Special Needs Guinea Pig Diets
Here is more information on IC:
Links - Interstitial Cystitis - Guinea Lynx Records
Thankfully so far, symptoms are on the mild side for your piggy.