Good luck at the vets. I really hope that your piggy is okay.
I lost my guinea pig to a UTI last year 8...
And it happened really quickly. I noticed that she wasn't eating her food and looked uncomfortable. I took her to the vet the next day, by which time there was blood in her urine. The vet put her on antibiotics, and she did seem to get a bit better the day after the vets visit (started eating and drinking). But went downhill over night. We were planning on bringing her back to the vet in the morning, but unfortunately she didn't make it.
But she was an old gal. We got her as a 3rd hand rescue, and the previous owner said she was at least 7 years old to their knowledge. She also had long fur, which I don't think helped much.
I now have two more rescue piggies, and I regularly trim their fur around their bums.