Bunny urine can sometimes get quite thick, depending on what the bun eats and how much water is ingested. Buns eliminate excess minerals like calcium through their urine. When the liquid part of the pee has trickled through the litter, it can leave the mineral sediment on the surface of the litter – it looks like mud, I guess. Having said that, if there’s too much calcium in a bun’s diet over a longer period of time, mineral deposit called sludge can accumulate in the bun’s bladder, making it more difficult for the bun to pee and leading to other issues. You can tell if it’s sludge usually – it has the consistency of toothpaste. Also, the bun will often posture and look like they’re making an effort when peeing. An xray usually shows sludge in the bladder and meds can be given and the bladder can be expressed by your vet. But I suspect that your bun just peed normal pee, since she’s so young.
I you want to be on the safe side, keep an eye on which fresh leaves she eats and cycle high calcium ones such as kale – and if she’s not a big water drinker, up her water intake by wetting all fresh food really well.