It might be stones but they really should have considered UTI. You take them in because you see symptoms and the symptoms are similar. Personally I'd go back and speak about antibiotics (and painkiller) as this thing hasn't gone away. Maybe your vet tests a urine sample first. You can put piggy in a plastic bowl until she pees to get a fresh one and collect into a little tube. Tbh mine doesn't test - she just hands out the ABs (usually 'baytril' over here) and offers an x-ray. They have a little poke around piggy's urethra and bladder area. If it's unusually tender or they can actually feel a stone they recommend the x-ray, but if not they will just say to try the AB and come back if the symptoms return.
When we've had stones we've also had UTI - and we've had UTI with no stones at all! My girls have had more plain infections than my boys: piggies scent mark by rubbing their bits against the ground and in girls especially this can encourage infection. They hide pain very well because they are a prey animal and they are notorious for pretending they are perfectly healthy when we take them to the vet (!) so it can be useful if you see symptoms at home to try to film them to show the vet that you're not just imagining these things!
The red flags for UTI/stones are blood in pee, wet bottom (in girls) or tummy (boys) from dripping urine, sometimes smelly, pain when peeing (you see them looking nervous, hunching posture and quietly squeaking as the pee). They might start to lose weight as pain can make them eat less. The bottom might start to look 'dirty' because muck gets stuck in the wetness. They will try to hold in the pee just like people with cystitis do - because it stings to go.
'Stones' can mean anything from a huge crusty spikey horror (although you'd know pretty soon if she was trying to pass one of those) to a mass of grit which makes peeing uncomfortable but does not impede the flow and she hopefully will be OK after it's come out. It's pretty normal for their pee to look a bit milky and leave a white 'shadow' as it dries out. If you see thicker patches of crusty grit that's a problem-in-waiting. One of my girls is greedy for pellets and had occasional bouts of crusty pee that I have to watch out for. I have water bottles dotted about to encourage drinking to keep everything flushing through. I have them right next to the hideys where they sleep so they don't have to come out into the open at night to drink... this has helped my shy drinkers to drink more. I have one very close to the pellet bowl as they will often take water while chewing the dry pellets. We checked the calcium in our tap water on the internet - it's very high in my area - a big clue is how disgusting the inside of your kettle looks! So we use bottled water with very low calcium. Filtering is also very effective at removing most calcium. Limiting pellets seems a shock if you are used to seeing a full bowl but try reducing bit by bit over time. That's what we had to do as my current pigs were still getting used to being together and I didn't want to stir trouble by changing too much too soon. I give pellets in the evening - in the day it's veggies, hay, grass, hay and more hay! The plan is that they're pretty full by the time the pellets show up.
But I would go back to the vet and ask them to consider UTI. My gritty pee girl has felt uncomfortable (for a couple evenings she looked gloomy and my other girl snuggled her) but she hasn't peed blood (touch wood). I've had stone pigs that have blood tint pee and one actually peed proper red - there was no wondering 'is that blood?' because it clearly was. But piggies with just UTI can pee blood tints. In truth it's the wet bottom that is the symptom I've noticed first though... maybe I should pay more attention! Good luck fluffy girl x
