Sorry, I forgot to parse in the mobility care link:
Looking after guinea pigs with limited or no mobility
The tips in the guide also work for any piggy not being well and not moving around lots; guinea pigs with strong urination/infection fluid are also at an increased risk of complications. I hope that the practical tips will help you.
If the antibiotic isn't working, the urinary tract is free of stones/sludge and the reproductive tract is fine, then your vet may look at a sterile interstitial cystitis, similar to a FSC (feline sterile cystitis). It is generally characterised by the presence of blood in the urine but a lower than expected bacterial count unlike a UTI, which is a bacterial urine infection caused by faecal bacteria getting into the urinary tract. Most piggies have some faecal bacteria in theirs because the tiny scent gland is located straight in front of the genitalia so bum wiping is a regular territorial behaviour in both genders. However, UTI can take over in piggies with a weak or lowered immune system.