Urine Intermittent pink urine

Hi guys, I'm freaking out here and I could really use some advice. Every week when I launder my guinea pig bedding I line their cage with white towels. Last weekend when I was taking the towels out to put the fleece back in I noticed two little pink spots on the towels. Each spot was very faintly pink and located in the center of a larger urine stain. Each spot was less than the size of a dime and neither spot had soaked through to the other side of the towel. I have a herd of three and no one has been behaving oddly. Everyone's weight is normal, no weird noises or strange behaviors. I gave everyone a pretty thorough inspection and nothing seems out of the ordinary.

Since I couldn't figure out who it was, I lined their cage with fresh towels and separated one of them to a holding cage also lined with white towels (figuring that I could at least narrow it down). But then it didn't happen again. I kept them on the white bedding for three days with no more pink spots. At that point I hoped that maybe it was just an isolated incident and switched back to their regular fleece.

Then tonight after I washed the linens I found two new spots! Again, they're very small, half the size of a dime each. Neither soaked through the towel and they were both faintly pink. One was in the center of a larger urine stain, the other was actually off by itself (no surrounding urine stain). So now I'm flipping out again. Does anyone have any advice for me? Or suggestions for how to figure out which one is sick so I can take them to the vet? If any of you live in Maryland do you have a good vet you can recommend? I live a bit south of Baltimore (right next to BWI actually).

Sorry about the wall of text, I'm panicking a little bit here. Thank you all in advance for any advice.