Hi and welcome!
Are you feeding more veg or more watery veg than normal? Please be aware that a guinea pig will only drink as much as it individually needs and not drink more because you provide more fluid in edible or drinkable form. You cannot force a piggy to drink more than it feels the need for - some piggies don't drink at all and others drink a whole bottle in a day.
If you haven't changed your veg, you can offer your piggy water by syringe once or twice daily, as much as it will take voluntarily in one session in order to make one large bladder flushing pee. Please never force any water down a piggy's throat just because you think they should drink more! Guinea pigs that are still eating normally don't usually dehydrate.
UTI (bacterial urine infection), bacterial cystitis (inflammation of the bladder walls), bladder stones/sludge and sterile (i.e. non-bacterial) interstitial (i.e. recurring) cystitis, or sterile IC, all present with very similar symptoms.
Your vet will generally work down the ladder from the most easily to cure bacterial infections (which react to antibiotics) to bladder stones/sludge (which require an x-ray) to a sterile cystitis, which is a persistent or recurring infection that cannot be cleared with antibiotics. Sterile IC cannot be healed, only managed with analgesics and glucosamine until it goes away on its own very eventually. Unfortunately it has become rather common over the last decade.
You can additional support the natural glucosamine coating of the walls of the urinary tract with additional glucosamine (we recommend a cat glucosamine based bladder food supplement like cystease capsules - cats are the other species prone to urinary tract problems) to help ease the dicomfort. Dissolve the contents of one capsule with 2 ml of water, shake well and wait until it has dissolved then syringe either the whole once daily or half twice a day.
Here are our diet tips for guinea pigs with with urinary tract issues:
Long Term Balanced General And Special Needs Guinea Pig Diets
Could you please tell us the medication details your piggy has been put on?
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Thank you!